Hello Leonfisher,
I am not aware of anything problem with using the WGA plugin in Firefox 3.6. I recommend going to
http://microsoft.com/genuine/diag and click the Start Diagnostic button. The site will ensure that your windows is setup to allow Validation (including having the Firefox Plugin installed) If the site is unable to resolve any problem it finds, automatically, it will provide manual steps that you can follow
If you still have problems, create a (no cost) WGA support request at
http://support.microsoft.com/gp/contactwga.
Thank you,
Darin MS