Can’t log in using Firefox

I am having the exact issue described in this thread:

http://wordpress.org/support/topic/149044

Firefox is *not* blocking cookies, and it’s not temporary because I’ve been trying to log in (in Firefox) for four days now. Firefox is the only browser I have had success performing all normal WP functions with – posting, editing theme, etc. – and now I can’t log in to my hosted blog using it.

Opera forces a reboot when I try to upload a header, Safari simply will not allow me to navigate Presentation and apparently does not support <p> breaks, and now this. I don’t think it would be melodramatic to say that this is making me strongly consider switching platforms.