Until someone gives a better answer look at: /questions/839921#answer-204635 and post back if you still have problems.

You honestly would think Mozilla would make sure many of these most used add-ons would work before releasing another version 3 months after releasing the LAST version. And.....you can't even go back to 4.0?

Thank you. Since I can go back to only the "3.6" (not "4"), which would not include Firefox support updates, I guess I'll stick with the already downloaded "5". I would assume McAfee and Firefox will eventually address and correct the problem.

I guess when you're in a rush to get new software into the system, it's easy to overlook the obvious. Please recall Microsoft brought out a updated Internet Explorer, a year or so ago, which was incomparable with most major websites. There fix was to install a button to activate the former compatible version of IE. So in reality you had the latest version on your screen, but were still using older version it was supposed to replace.

The McAfee Site Advisor 3.3.1 extension was the number 2 cause of crashes in one of the Firefox 5.0 betas and Mozilla Blocklisted it. That extension is disabled in Firefox 5, Firefox 6.0beta, and Firefox 7.0alpha versions. McAfee is working on a new re-written version of the Site Advisor extension to fix their problems - ETA is mid to end of July.
https://bugzilla.mozilla.org/show_bug.cgi?id=660111

credit to the-edmeister

McAfee have taken the decision to rewrite the Firefox SiteAdvisor plugin to make it easier to update. This should ensure that Mozilla's fast-release schedule does not cause lengthy delays after future version updates. What it means for Firefox now though is that until the rewrite is complete - and tested - SiteAdvisor is being prevented from loading in Firefox in order to avoid the possibility of software incompatibility.

The SA rewrite is projected to take a few weeks to complete and test; it should be ready by about the third week of July, although the exact date is not being given at this stage. It could be earlier.

In the meantime, users who are concerned about the safety of websites may wish to consider WOT as a temporary substitute. WOT ratings are set by user consensus rather than by automated site testing, so WOT is not a full replacement for SiteAdvisor. It is, however, a good rating tool in its own right (I use both it and SiteAdvisor in all my browsers) and is apparently unaffected by the upgrade to FF5.

There is a discussion about this on the McAfee forums HERE

Thank you. I'll wait for the problem to be corrected.