Rated 5 out of 5 stars

If you go to the author's website and install the extension from there, it will work. The install.rdf file allows a max version of 3.5.* for Firefox.
I'm guessing they patched the file but forgot to update the date/time stamp. And upload the changes to Addons.

Anywho, it works fine for me with Firefox 3.5.3!

http://www.asnumber.networx.ch/

This review is for a previous version of the add-on (1.0Beta9).