It is blocklisted by Mozilla due to that Site Advisor extension being the #2 cause of crashing the Firefox 5.0 betas. McAfee was notified of the problem long before that extension was added to the blocklist. Now it is up to them to fix it.
https://addons.mozilla.org/en-US/firefox/blocked/

I have read that McAfee is working on a re-written version that is projected to be available in the 3rd week of July - which has come and gone. Check with McAfee for availability of that new version of the Site Advisor add-on.

So why didn't you folks advise us that 5.0 is a beta version. If I had known that I would not have updated to the beta. How do I get back the previous version?