<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:bd953243-b5fc-4a41-9c19-529e2fe7efef,g:0cf06768-1603-4a17-adb0-607cbcac2839-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.f-e-t.com/role/CoverPage" xlink:type="simple" xlink:href="fet-20260430.xsd#CoverPage"/>
  <link:definitionLink xlink:role="http://www.f-e-t.com/role/CoverPage" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentInformationTable_e3513677-3689-4320-8f91-966e44df1ae3" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentInformationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_DocumentInformationTable_e3513677-3689-4320-8f91-966e44df1ae3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityListingsExchangeAxis_3f665ff7-f176-4f98-b3ac-99c8cd242c03" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityListingsExchangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_e3513677-3689-4320-8f91-966e44df1ae3" xlink:to="loc_dei_EntityListingsExchangeAxis_3f665ff7-f176-4f98-b3ac-99c8cd242c03" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_3f665ff7-f176-4f98-b3ac-99c8cd242c03_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_EntityListingsExchangeAxis_3f665ff7-f176-4f98-b3ac-99c8cd242c03" xlink:to="loc_dei_ExchangeDomain_3f665ff7-f176-4f98-b3ac-99c8cd242c03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_ExchangeDomain_613c9148-7685-490c-af30-a1523f2c4c75" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_ExchangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_EntityListingsExchangeAxis_3f665ff7-f176-4f98-b3ac-99c8cd242c03" xlink:to="loc_dei_ExchangeDomain_613c9148-7685-490c-af30-a1523f2c4c75" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_fet_Exch_XNYSMember_a14b6b5e-7d57-4ebf-9354-b5658e8a9a05" xlink:href="fet-20260430.xsd#fet_Exch_XNYSMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_ExchangeDomain_613c9148-7685-490c-af30-a1523f2c4c75" xlink:to="loc_fet_Exch_XNYSMember_a14b6b5e-7d57-4ebf-9354-b5658e8a9a05" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_fet_Exch_XCHIMember_c02d7b7d-43b7-4831-887c-42a54943dabb" xlink:href="fet-20260430.xsd#fet_Exch_XCHIMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_ExchangeDomain_613c9148-7685-490c-af30-a1523f2c4c75" xlink:to="loc_fet_Exch_XCHIMember_c02d7b7d-43b7-4831-887c-42a54943dabb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentType_c9c11ede-a925-4b2f-9a5d-454c81609670" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentType"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_DocumentType_c9c11ede-a925-4b2f-9a5d-454c81609670" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_DocumentPeriodEndDate_bee50dd1-f67c-4627-8008-e320b8c1636f" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_DocumentPeriodEndDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_DocumentPeriodEndDate_bee50dd1-f67c-4627-8008-e320b8c1636f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityRegistrantName_17eede73-ddbb-4c75-b91c-26813db13579" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityRegistrantName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityRegistrantName_17eede73-ddbb-4c75-b91c-26813db13579" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityIncorporationStateCountryCode_3bcf7553-1ed9-430a-bbd4-8cc9de2efb32" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityIncorporationStateCountryCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityIncorporationStateCountryCode_3bcf7553-1ed9-430a-bbd4-8cc9de2efb32" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityFileNumber_8dda1e81-c36b-40da-bab7-d63c9c0bbe31" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityFileNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityFileNumber_8dda1e81-c36b-40da-bab7-d63c9c0bbe31" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityTaxIdentificationNumber_c93b8be6-2674-46da-9e78-28697edcad31" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityTaxIdentificationNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityTaxIdentificationNumber_c93b8be6-2674-46da-9e78-28697edcad31" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine1_8f381b4d-4e4e-4347-b50e-6d9ab416805b" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityAddressAddressLine1_8f381b4d-4e4e-4347-b50e-6d9ab416805b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressAddressLine2_005f3110-1052-4a93-8ee5-2eed74d8b628" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressAddressLine2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityAddressAddressLine2_005f3110-1052-4a93-8ee5-2eed74d8b628" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressCityOrTown_7820c0ac-5205-49b8-8d53-96d26e0fd301" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressCityOrTown"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityAddressCityOrTown_7820c0ac-5205-49b8-8d53-96d26e0fd301" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressStateOrProvince_318dfe41-c6c5-405d-b716-31a2511ce6d2" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressStateOrProvince"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityAddressStateOrProvince_318dfe41-c6c5-405d-b716-31a2511ce6d2" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityAddressPostalZipCode_045eb2c4-2e6a-40df-8c9e-88e454d30a63" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressPostalZipCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityAddressPostalZipCode_045eb2c4-2e6a-40df-8c9e-88e454d30a63" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_CityAreaCode_51414a00-a711-4617-92c4-490e7e2cbd34" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_CityAreaCode"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_CityAreaCode_51414a00-a711-4617-92c4-490e7e2cbd34" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LocalPhoneNumber_69b22cc9-b63c-4287-b38c-d88f950ae218" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LocalPhoneNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_LocalPhoneNumber_69b22cc9-b63c-4287-b38c-d88f950ae218" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_WrittenCommunications_c1ba774d-22db-484b-86c7-851e8ee54b0e" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_WrittenCommunications"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_WrittenCommunications_c1ba774d-22db-484b-86c7-851e8ee54b0e" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SolicitingMaterial_ad9517c5-9f47-427e-bcdb-0d64dba9fee7" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SolicitingMaterial"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_SolicitingMaterial_ad9517c5-9f47-427e-bcdb-0d64dba9fee7" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementTenderOffer_8ec4e487-2421-4d09-af87-127f393d7344" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_PreCommencementTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_PreCommencementTenderOffer_8ec4e487-2421-4d09-af87-127f393d7344" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_PreCommencementIssuerTenderOffer_6b0e3070-78c6-45a9-a927-5ad96fb8db41" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_PreCommencementIssuerTenderOffer"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_PreCommencementIssuerTenderOffer_6b0e3070-78c6-45a9-a927-5ad96fb8db41" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_Security12bTitle_9f80e0d6-f91e-42ae-8d75-af9955f42712" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_Security12bTitle"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_Security12bTitle_9f80e0d6-f91e-42ae-8d75-af9955f42712" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_TradingSymbol_61f8cfbb-472b-44b1-8a38-eefdd1b8d3c9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_TradingSymbol"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_TradingSymbol_61f8cfbb-472b-44b1-8a38-eefdd1b8d3c9" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_SecurityExchangeName_12c9a412-c38c-42c2-a57f-8a8d8eb6ce81" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_SecurityExchangeName"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_SecurityExchangeName_12c9a412-c38c-42c2-a57f-8a8d8eb6ce81" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityEmergingGrowthCompany_51da9634-5fe3-4d28-9ffa-00cea14f9586" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityEmergingGrowthCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityEmergingGrowthCompany_51da9634-5fe3-4d28-9ffa-00cea14f9586" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityCentralIndexKey_d4ebaa32-90ef-4f8a-aeb2-ba3431685bbf" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityCentralIndexKey"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_EntityCentralIndexKey_d4ebaa32-90ef-4f8a-aeb2-ba3431685bbf" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_AmendmentFlag_2c2f328c-8ba0-4620-9c5f-5769d3aee304" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AmendmentFlag"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_29c5a783-dbd8-4317-997e-479e04a3ade5" xlink:to="loc_dei_AmendmentFlag_2c2f328c-8ba0-4620-9c5f-5769d3aee304" xlink:type="arc" order="22"/>
  </link:definitionLink>
</link:linkbase>
