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

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2022-07-19</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001145921</rptOwnerCik>
            <rptOwnerName>SOMERHALDER JOHN W II</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>76 SOUTH MAIN STREET</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>AKRON</rptOwnerCity>
            <rptOwnerState>OH</rptOwnerState>
            <rptOwnerZipCode>44308</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2022-07-19</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>745</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>37.775</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>24066</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>416.046</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Savings Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Reflects prorated second quarter compensation for John W. Somerhalder II  approved by the FirstEnergy Corp. Board of Directors on July 19, 2022. Mr. Somehalder's compensation is in recognition of his service as non-executive Board Chair and was retroactively effective as of May 21, 2022.</footnote>
        <footnote id="F2">FE's 401(k) Savings Plan includes a unitized fund invested in FE stock, in which the reporting person may invest, and includes dividend reinvestment and company match features. The number of shares reported as indirectly held in the 401(k) Plan in this row is an estimate of the number of shares of FE's common stock held in the unitized stock fund and allocated to the reporting person's account as of June 30, 2022.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Mary M. Swann, attorney-in-fact</signatureName>
        <signatureDate>2022-07-21</signatureDate>
    </ownerSignature>
</ownershipDocument>
