I was having a problem with Firefox 3 where all my cookies were being deleted each and everytime I re-launched the program. Luckily after some Google-fu, I found some other people having similar problems to my own. The solution? Deleteing the Torbutton extension. Seems to have solved my problems.
I’ve been the good boy that WordPress wants me to be and I’ve just upgraded to 2.6. It wasn’t without its moments, though. All the upgrade process went just fine, but when I went to log in to my dashboard–the backend of the blog–I was met with a message that said that my password was wrong. I tried re-setting the password twice with no avail. Luckily, I found this post that described the same problem I was having (with the exception that this person was using Safari rather than Firefox in my case). The suggested solution worked for me: clear my browser cookies & cache. So, I tossed my cookies, cleared my cache, closed down Firefox, re-launched the browser and, voilà , I was able to log-in with my newly-reset password.