Is there a known problem with Firefox & Multisite?

I've been having trouble all day with a new multisite not allowing me to log in and it was just driving me crazy. This was using Firefox v3.6.6

I decided to try out Opera and it all worked fine.

I know that this new version of Firefox on the PC does not load any pages that use Google Analytics, but are there more problems that I've not come across before now that you know of?