On my Intel Mac, I went back to Firefox 3.6.19, that way I can keep using the Site Advisor plugin until the Firefox 5 issues get resolved. Also, if you aren't using K-9 content filtering, you can also get the AVG Link Scanner (similar to McAfee Site Advisor) which also works on Safari and Firefox.
How I uninstalled Firefox 5 was to do a search for any folders named 'Firefox' or 'Mozilla'. Then delete those, and download the 3.6.19 installer. Haven't noticed any security problems so far...
Thanks for providing the link to SiteAdvisor 3.4 beta. I downloaded it but it's taking forever to install. It says "Please wait while we install your software" and the circle spins but nothing else happens. It has been doing that for 10 minutes now and I don't think it should be taking this long, especially for an approx. 7 MB file.