There have been service outages problems with servers on the Add-ons site. If you still have the problem then try again later.

I do not know how but i succesfully have upgraded now to 9.01. I'm not sure how i did it, but it was after i sent te question to you.

Thanks if that had anything to do with it. Otherwise it was pure luck from my site because i have not enough knowhow about computers

I have been using version 3 for years, as every time I tried an update, it would hang. Same with 9. So I always had to go back and reinstall 3, where everything would be fine. 9 works on my laptop with Vista, and the new feature I wanted was the sync, to be able to sync bookmarks. Love that idea. But on my desktop with XP, could not for the life of me get Firefox to work with any version past 3.

Here's the solution that worked for me:

   opened the version that worked, and saved bookmarks to USB drive. That's the main thing I wanted in the new version.
   uninstalled FF
   manually deleted all files in FF and Mozilla folders on the hard drive, including any folder named "mozilla" or "firefox"
   edited the registry (read about how to do this properly elsewhere) and manually did a search for and removed every single instance of "mozilla" and "firefox" that existed on the computer. Made sure I did search from the top of the registry tree
   rebooted
   installed firefox 9
   imported bookmarks from USB drive 

all works fine now! Years of upgrades not working, and a registry clean did the job.