Kibeki,
it looks to me like the problem is at your end.
I've just upgraded a windows pc to firefox 3.6 and then installed Noojee Click from our web site.
The file downloaded and installed without a problem.
My best guess is that you have a corrupt version either in your browser or maybe your companies proxy server.
One thing you can try is installing the plugin directly from the mozzila website.
goto:
https://addons.mozilla.org/en-US/firefox/search?q=noojee&cat=all&advancedsearch=1&as=1&appid=1&lver=3.6&atype=0&pp=20&pid=5&sort=&lup=and download that version (its identical) which should bypass your cache issue.