After installing the latest Firefox, I have found a major functionality bug when trying to close tabs.
Whenever I click to close a tab, the page remains, and when I select another tab, I cannot return the tab to focus, nor can I close it out. Apparently the tab is "closed" but it is also still "open"; the tab still lingers on the tab bar after clicking the close button on the tab. When I go to click the tab after I've clicked another unclosed tab adjacent to it, it selects the tab to its right, unless it's the last tab. Sometimes it appears as though it is indefinitely loading the page it was on. I've found that I can only clear these bugged-up tabs for a short while after restarting Firefox. I am also running this on Ubuntu 10.04 LTS.
Edit: After restarting I am able to close the tabs, as they've remained open after closing the browser; however new tabs that I've opened afterwards continue to suffer the bug. Furthermore, when a tab has bugged up in the way I mentioned, right-clicking and selecting "Undo Close Tab" brings back the "closed" tab in a new tab, which I can close, but does not close the stuck tab. Also, in Panorama mode, I found that I can switch back to these buggy tabs, but I still cannot close them, and if I click away from them, I cannot return to them, unless I switch to them in Panorama. I also cannot close them in Panorama mode. however, I can drag them into a new tab group, but this doesn't solve the problem much, especially considering that other open tabs continue to not-close.
Wot anima42
Wšě wotmołwy (2)
This might be a possible fix for some users, especially those who have the Tabberwocky add-on installed to their browser: after removing Tabberwocky, tabs started functioning normally again for me. This is probably an issue with the Tabberwocky add-on, as it has not been updated yet for Firefox 4.0 (at the time of this writing the latest it works for is 3.5 to 3.7a3pre). I don't know if this may be the case for other users experiencing this problem, but for those that do have the Tabberwocky add-on, disable it first to see if it's causing the problem for you.
Wot anima42
This seemed to come out of the blue, very much as described by anima42. I also am a Ubuntu 10.04 user. I have pondered on the first reply above, and though I don't, and didn't, use Tabberwocky, I was using Tab Mix Plus. Having removed this, the problem seems to have gone away. Another possible clue as to what went wrong: I use a constant Firefox profile located in my home directory. I dabbled with Ubuntu 11.04 a week or so ago, which runs (I think) a more updated Firefox. This Firefox was operating on my default profile, as you would. It may have broken something so it now doesn't play nicely with the 10.04 setup. Just surmising.