I just had a very odd incident with my session locking up, so I thought I'd describe it to see if it rings any bells with anyone.
Background: I shut down my machine last night for Earth Hour, so the last thing I did before logging out was exit Firefox3, telling it to save my open tab state. When I turned the system back on and started FF3 again, it installed new versions of three plugins (I believe NoScript and Elasticfox but I don't recall the third) and then restored the session as expected.
Not related to FF, but I also started up the "filelight" filesystem visualization app, which I don't normally have running, and it was still up on a different desktop while all the following was happening.
This morning I opened a new FF3 window to browse CNN. I opened tabs on five or six stories from the front page and then went to read through them, closing each tab as I finished each story. (There were six other tabs open in the first FF window, so no more than 12-14 tabs in two windows total.) After closing the second such tab FF began responding very slowly, even to simply scrolling the window, and eventually stopped responding entirely (wouldn't even redraw when I dragged windows around). I popped up an xterm and ran "strace" on FF to see if I could find the problem, and lo and behold as long as strace was running FF recovered and responded fine, but when I stopped tracing it froze again. Unfortunately I lost the trace output because of what happened next, but I do recall that it was accessing /etc/localtime repeatedly and very frequently.
I left strace running and went back to FF, hoping to see the tail of the trace at the moment FF locked up again. However, the next time FF appeared to be frozen I found that not just FF but the entire Gnome session was stuck. I was able to move the mouse and to Alt-F2 into a text console window, but nothing else responded. I went back and forth a few times between the text console and the X console attempting to diagnose (including killing the strace and sending a CONT signal to FF, because "ps" said FF was in stopped ("T") state), but after a few passes at that even Alt-F2 stopped working.
Fortunately I have another computer here, so I ssh'd in and ran "killall -1 firefox-bin". This woke things up to the point that one of the pending Alt-F2 keystrokes was seen and the text console came up, but although I could then switch consoles again, nothing else in the X session was responding. I finally HUP'd gnome-session (which stopped everything else; I checked "ps" to make sure nothing that had been associated with that session was still running) and I have now logged in again and everything seems fine. FF restarted and restored the session from the time I killed it, and an strace of it now doesn't show any accesses to /etc/localtime at all.
I find this all especially strange because the whole system had been restarted less than 12 hours before.