disable User Timing API in ESR60
In legacy/trac#16336 (moved), we disabled the User Timing API by setting
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.