> Polyinstantiated /tmp should make your problem go away.
In fact, I just patched the source code (a slightly different patch than the one in that particular bug report). Not knowing the mozilla source code, it took me nearly a day to track this bug down (not having found the earlier bug report in bugzilla), and I ended up being really surprised that such a basic error, with such a trivial fix, and with user impact, could be allowed to linger so long in the code base, particularly when I later found out that there was an active bug report from 2010.
That is as it may be. For those who don't want to patch the source, I suspect it would be easier to start firefox with a shell script which sets TMPDIR.