Comment 26 for bug 308605

You make Firefox create a new profile, then move all old contents to the new one. This most probably removes a "lock file". When Firefox starts, it notes in the profile "this one is already in use", so a new Firefox process doesn't corrupt that profile by using it twice at the same time. You're working around that by creating a new profile, then moving the contents open, thus making it possible for Firefox to unintentionally destroy your data.
Just use the workarounds in this bug report: Run "killall firefox" in your terminal so it dies again.

For the purpose of this bug report: I haven't had this bug in ages. Maybe the bug was in KDE/Qt (since that shows up in the backtrace) and it has since been fixed. On the other hand, sometimes it seems to be caused by other things, like pulseaudio and maybe flash: when a firefox instance is still around, killall pulseaudio kills it too, iirc. Any other known causes for this bug to appear? If not, I think the bug is safe to close.