I had this problem as well...
What I found that fixed it was a) clear cookies/cache/tif
and b) when you go to post something, make sure that your password is in the "password" field. For the longest time, whenever I would try to post with Firefox, it would randomly not enter my password in, despite the fact that I was logged in.
---
Okay, Lindsay, are you forgetting that I was a professional twice over - an analyst and a therapist. The world's first analrapist.