Vista 'deciding' a program needs to be run as administrator

Hi, I'm using Vista Home Premium, and up until now everything's been going
fine. Today I decided to install an add-on for firefox, which worked fine,
but when I closed firefox Vista popped up a dialog suggesting that something
may not have worked correctly, and asking if I would like to try running
firefox with administrator priveleges. Thinking that this would be one-off,
perhaps to complete installation of the addon, I pressed yes. Now every time
I run firefox I get the UAC 'Continue' screen. I've looked in compatibility
settings, and nothing's checked.
I did a little experimentation and found that if i renamed or moved
firefox.exe, this no longer happaned. In fact, if i replace firefox.exe with
another completely unrelated executable (say notepad.exe), vista will run it
as administrator, ie. Any file called C:\program files\mozilla
firefox\firefox.exe now requires administrator priveleges to execute. How
can I resolve this issue?
A web search returns one forum posting with the exact same problem, on the
MSDN forums, and these guys dont seem to solve the problem either:

http://channel9.msdn.com/ShowPost.aspx?PostID=302717

And another thing: I initially thought "well, since I spent so much on vista
I may as well use the professional support", but on going to
support.microsoft.com and following the instructions, I'm told that I
entered my product ID in an incorrect format, even though I did not. It even
says this when I allow the ActiveX control to submit my ID for me.

Thanks in advance for any ideas.