After some discussion with jcristau and mythmon: It sounds like our best bet to flip the pref would be via a traditional patch-uplift, which we could ship as a ridealong if there's a dot release to fix some other bug. (Normandy is also sort-of an option that doesn't involve shipping an update, but it turns out that approach isn't currently well-suited to turning off a pref on just a single release.) So: if we don't end up having a dot release, then we'll have to live with this bug for a few weeks until Firefox 86 ships to release, which will hopefully include the real fix (mats' attached patches).
Bug 1689789 Comment 32 Edit History
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After some discussion with jcristau and mythmon: It sounds like our best bet to turn off the pref would be via a traditional patch-uplift, which we could ship as a ridealong if there's a dot release to fix some other bug. (Normandy is also sort-of an option that doesn't involve shipping an update, but it turns out that approach isn't currently well-suited to turning off a pref on just a single release.) So: if we don't end up having a dot release, then we'll have to live with this bug for a few weeks until Firefox 86 ships to release, which will hopefully include the real fix (mats' attached patches).