I've noticed this a few times on some browser prompts (eg HTTP auth), seems like it's regressed within the last few weeks sometime? 1) Load http://dolske.net/mozilla/tests/prompt/onbeforeunload.html 2) Click back to trigger native prompt 3) Note that "Stay on Page" and "Leave Page" are both gray buttons 4) mouseover the default (Leave Page) 5) It suddenly turns blue with a (janky) animated pulse. Expected: The color/animation from (5) should start immediately upon the prompt being shown. The animation is also rather janky (compare to Safari), not sure if that's a separate bug. I don't remember it being bad before?

Last good nightly: 2013-03-07 First bad nightly: 2013-03-08 Pushlog: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=ee4879719f78&tochange=cb432984d5ce Almost certainly bug 839029, which leads me to bug 892547.