User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:23.0) Gecko/20130518 Firefox/23.0 (Nightly/Aurora) Build ID: 20130518004019 Steps to reproduce: The most recent versions of Aurora have added native-looking overlay scroll bars to the Mac OS X version. These appear as expected when the page is at default zoom level, but if the zoom is increased, the scroll bars zoom in too. Steps to reproduce: 1. Visit any scrollable page on Aurora for Mac. 2. Scroll up and down, noting the native-looking size/appearance of the overlay scroll bars. 3. Zoom the page in several times with ⌘+. 4. Scroll up and down, noting the now over-sized and (in some cases) glitchy-looking scroll bars. Actual results: Scroll bars increase size with page zoom. Expected results: Scroll bars should always stay the same size, independent of page zoom. This is the behaviour of all native OS X applications which feature a zoom function (graphics editors, word processors, &c.), including Safari and Chrome.
I've used mozregression and find that Last good nightly: 2013-05-02 First bad nightly: 2013-05-03 Pushlog: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=70e0955ccc87&tochange=b35170667a2f
Oh, in following some of the links from that page, it looks like this is actually a duplicate of bug # 868416.