Every time I try to run Firefox on one of my computers it brings up a UAC dialog and I can't work out why.  I've tried uninstalling and reinstalling firefox (more than once) but that doesn't help, and even when I try to run Firefox in safe mode it still triggers UAC.  Does anyone have any idea what's causing this and how I can stop it?

Thanks.

Windows Vista Utlimate 64bit
Firefox 2.0.0.3 English (British)