disable User Timing API in ESR60

In #16336 (moved), we disabled the User Timing API by setting dom.enable_user_timing = false. Support for this pref was removed as of Firefox 55; see https://bugzilla.mozilla.org/show_bug.cgi?id=1344669.

We should do patch Firefox to resurrect support for this pref or do something else to mitigate the fingerprinting potential of this feature.

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