EDIT: Just realised this is from over 2 years ago (facepalm)
OK, I have a couple of idea's that may or may not help:
1 - Click the Firefox Button, and click Options. Click the Privacy Tab. And change the Keep Until to Forever or whatever. Now click Show Cookies:
A new popup should open. Click "Remove all cookies".
2 - If this fails, try this:
Click Firefox Button > Options > Menubar. Now click Tools and chose "Clear Recent History..." or instead press CTRL/Shift/Delete.
Another popup should open.
Change the time Range to Everything and Check Every Checkbox and select "Clear now".
3 - If this still fails, try creating a new Firefox Profile.
Warning
If you create a new profile and delete the old one, you will lose addons, history and bookmarks, so please back them up
Completely exit Firefox, use Task Manager if necessary
Open a Run Box, I am sure you know how, so I am not going into Detail and type in the run box "firefox.exe -P". Click OK or press Enter.
On the first popup chose next:
Now press "Create Profile":
Type in the Profile Name you chose: I have Chosen Test.
Now Chose Finish.
Feel free to delete your other Profiles while your still at it.
Chose your recently created Profile and chose Start Firefox:
However if you did chose to delete your profiles, you should see this:
Sorry for such a long post but I hope this helps.
