I have the same thing. I use FF for everything but now use Chrome for my Ning site. FF seems very buggy lately and I crash it with a ton of windows open.
I should have checked different browsers first but I recently added code and thought that may have been the culprit.
Thanks guys for a speedy response!
FYI: We've ratcheted up the priority on FF 7 fixes, so hopefully we can get this fixed for you soon.