Firefox stopped working after user agent switching update

Hi all, 

I have firefox version 1.0.2 installed on windows XP. 
I accepted an automatic update of an extension (user agent switching).
Firefox is now behaving very strangely : 
When I start it up, it loads up but does not get my home page. I can't
access the menus any more. The drop down menus appear all over the
screen and not right below the menus.
Basically Firefox does not respond to any clicks or keyboard shortcut.
The only icon that works is the "check for update" icon. But no update
is available.

I have tried uninstalling, deleting the Mozilla Firefox directory,
reinstalling.
Still the same. 

Thanks for your help. 

PY