Visit the URL. Drag something (the given list elements, or a local file, or whatever) onto the trash can. It becomes colored while the cursor is over it, and loses its color when the cursor leaves. Now press Esc with the cursor still over the trash can to cancel the drag; the color remains. The page has a very simple dragleave handler to remove the class that switches to the colored image. It's not being called at all when the drag is canceled. This seems to be a regression: dragleave *is fired* in Fx 3.6 on WinXP/OSX. dragleave is *not fired* in Fx 7 beta on Linux/WinXP/Win7 and Fx 5.0.0 on WinXP. I haven't had a chance to narrow it down further.
Regression window(cached m-c hourly) Works: http://hg.mozilla.org/mozilla-central/rev/01fa971e62ee Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b5pre) Gecko/20100827 Firefox/4.0b5pre ID:20100827190212 Fails: http://hg.mozilla.org/mozilla-central/rev/0886ad6e6aaa Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b5pre) Gecko/20100827 Firefox/4.0b5pre ID:20100827190555 Pushlog: http://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=01fa971e62ee&tochange=0886ad6e6aaa In local build: built from 4722b491cd0d : works built from 687b70fea4d0 : Fails built from 7804b5cf4313 : Fails Triggered by: Bug 130078 - integrate iframe into chrome view hierarchy (link view managers / trees between chrome and content)
Mentioning bug 481737, but I don't know whether or not they both need the same solution.
Given the culprit, bug 591249 sounds strangely relevant, though it existed in 3.6.
Should be fixed by bug 591249.