I'm using a socks 5 proxy (tor). This happened on 2 amd64 machines. When browsing pages firefox will exit at random times for no apparent reason. This causes the clipboard to be cleared (like when any GTK app closes) and all sessions are lost. It always seems to be during user interaction (maybe only when loading pages) and not while idle. There are no segfaults/OOM etc in kernel log. Reproducible: Didn't try
The 2 machines it happend on had www-client/firefox-10.0.7:0 and www-client/firefox-10.0.9:0
You need to run it via strace -f firefox then post a log here. Also emerge info is needed on every bug report. Please set to unconfirmed soon as info requested is attached.