I'm told (in bug 1201354, comment 7, and see bug 1201354, comment 9) that running Firefox in a Linux root account is unsupported. I assume that fixing that is complicated, although we may need Firefox while in root. However, even if changing that is too complicated, at least tell us it's unsupported. For example, when FF starts, have it detect if it is in the root account and, if so, display a message about being unsupported (and probably recommending the use of any nonroot account, which root users should have anyway). Otherwise, we'd legitimately assume that whatever FF does or gets is the same as in a nonroot account and that if it obviously fails it must be for some reason other than our choice of an all-powerful account. If this involves su (etc.) as well, the same or a similar solution should be applied.

Bug 1323302 has been resolved, and Firefox should no longer start when run as root. yay!