User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b4) Gecko/20100818 Firefox/4.0b4 Build Identifier: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b4) Gecko/20100818 Firefox/4.0b4 In the latest beta (I think it didn't appear before 4.0b4), when you close a tab it now performs the unusual animation of rapidly "shrinking/sliding" the tab into the previous one. That's okay, except that sometimes it gets stuck. See the URL for this specific instance. In this case, I closed a tab named "Cisco SR2016 expert reviews", and it did the close tab animation, but left the favicon hanging between two other tabs... This is still counted as a tab. It has a name, it can be switched to, it can be saved in add-ons like Session Manager, and memory usage appeared to be the same after closing... I don't know why it didn't complete the tab-closing process, but anyway, there you have it. Problem may be caused by add-on "Tab X" which was fully compatible with the beta thus far, but maybe something has been slightly changed with tabs that rendered it incompatible. Reproducible: Sometimes Steps to Reproduce: 1. Open a lot of tabs, particularly ones with favicon data. 2. Close various tabs rapidly. 3. If it doesn't work, install Add-On Compatibility Reporter, and then Tab X, enable Tab X, and try again. Actual Results: Tab remained in limbo, neither living nor dead Expected Results: Tab should have been closed Using custom theme, but the problem isn't really related to graphical glitches.

Whoops! Sorry for creating a duplicate; you must admit that to a non-code-insider, the title of the other thread would be difficult to recognize as the same issue :) Anyway, this can serve as another confirmation to it...