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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2011-05-10</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001221442</rptOwnerCik>
            <rptOwnerName>JOHNSON JULIA L</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>76 SOUTH MAIN ST</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>2011-05-10</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>509</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>42.269</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>18506.087</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                    <footnoteId id="F1"/>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Phantom Stock</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <footnoteId id="F3"/>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F4"/>
            </exerciseDate>
            <expirationDate>
                <footnoteId id="F4"/>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>7065.039</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>7065.039</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Upon completion of the merger pursuant to which Element Merger Sub, Inc., a wholly owned subsidiary of FirstEnergy Corp. (&quot;FirstEnergy&quot;), merged with and into Allegheny Energy, Inc. (&quot;Allegheny Energy&quot;) with Allegheny Energy becoming a wholly owned subsidiary of FirstEnergy (the &quot;Merger&quot;), FirstEnergy issued to Allegheny Energy stockholders 0.667 of a share of FirstEnergy common stock for each share of Allegheny Energy common stock held prior to the merger.  On the effective date of the Merger, the closing price of FirstEnergy's common stock was $37.75.</footnote>
        <footnote id="F2">In accordance with the terms and conditions of the FirstEnergy Corp. Deferred Compensation Plan for Outside Directors.</footnote>
        <footnote id="F3">Each share of phantom stock is the economic equivalent of one share of FirstEnergy Corp. common stock.</footnote>
        <footnote id="F4">The phantom stock units were converted upon the FirstEnergy Corp./Allegheny Energy, Inc. merger based on an exchange ratio of 0.667 of a share of FirstEnergy Corp. common stock for each share of Allegheny Enercy, Inc. common stock and are payable in cash upon separation/termination from FirstEnergy Corp. Board.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Edward J. Udovich, POA</signatureName>
        <signatureDate>2011-05-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
