<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2023-03-01</periodOfReport>

    <dateOfOriginalSubmission>2023-03-03</dateOfOriginalSubmission>

    <issuer>
        <issuerCik>0001031296</issuerCik>
        <issuerName>FIRSTENERGY CORP</issuerName>
        <issuerTradingSymbol>FE</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001322841</rptOwnerCik>
            <rptOwnerName>Park Hyun</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>76 S. MAIN ST.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>AKRON</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>44308</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isOfficer>1</isOfficer>
            <officerTitle>SVP &amp; CLO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2023-03-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>11027</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>38.9475</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>21433</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Represents the vesting of the 20PSUS20 award. The award vested on March 1, 2023. The shares coded &quot;F&quot; were automatically withheld to cover tax obligations associated with the payout. Amounts also include dividend reinvestments.</footnote>
        <footnote id="F2">The original Form 4, filed on March 3, 2023, is being amended by this Form 4 amendment to correct an administrative error that incorrectly reported the price of securities disposed of in Column 4 as $39.9475. All other information in the original Form 4 was correct and remains unchanged following this amendment.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Daniel M. Dunlap, attorney-in-fact</signatureName>
        <signatureDate>2023-03-06</signatureDate>
    </ownerSignature>
</ownershipDocument>
