As of the 19th or so, LOTS of Firefox users have not been able to use PHP based websites (example: SMF based forums)
This issue seems to be a Windoze only issue.

EDIT:
There is an annoying little trojan floating around that exploits FF's downloader
it installs 2 reg startup keys
I used CCleaner to fix it
Check Tools>Startup for 2 keys: the first is random jibberish, and the second is 'BM ' followed by the same jibberish
Delete both and restart. Done