Created attachment 511198 [details] Screen recording of problem. According to dolske and fryn, this is a regression. Have not tested platforms other than Mac to see if the problem shows up.
Similarly to bug 616010, the UI does not jitter if already wearing a persona.
Tested out on nightlies, works in 02-04 (847a825087f2) but fails in 02-05 (4574f9e56d49). The corresponding changeset: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=847a825087f2&tochange=4574f9e56d49
Luckily there are still tinderbox builds. Narrowed it down to the change: http://hg.mozilla.org/mozilla-central/rev/51f36c6a7d38 from bug 627328
Created attachment 511235 [details] [diff] [review] Patch v1, switched to using a script runner. From reading bug 627328, it seemed like a script runner was thought of but never used. Tested this out and it solves the problem on Mac, going to run try builds to check other platforms.
Should be a hardblocker IMO, the effect is nasty.
Comment on attachment 511235 [details] [diff] [review] Patch v1, switched to using a script runner. r=me