chore: sync Firefox contentblocking preference change from Bug 1977066

[juliandescottes]

Expected value of this preference changed since Firefox 139, but this specific usage was missed (see https://bugzilla.mozilla.org/show_bug.cgi?id=1977066).

Initial change landed in https://bugzilla.mozilla.org/show_bug.cgi?id=1956556

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[jennifer-shehane]

@juliandescottes Thanks for the contribution! Could you please sign our CLA?

[jennifer-shehane]

We'll have to close the issue without a CLA signature.