Try this:
1) Quit Firefox.
2) Goto Start->Run
3) Type 'cmd' - click ok
4) Paste this: "c:\Program Files\Mozilla Firefox\firefox.exe" -p
This should start firefox with the profile dialog. Create a new
profile, call it whatever you want and then start Firefox with that
new profile.
5) Install Jeteye plugin - you can get the latest version here:
http://downloads.jeteye.com/jeteye-1.6.5.2.xpi
Try and login. This will be as clean an installation as you can get.
Let me know how that works.