Firefox 5.0 users are not getting updates to 6.0 Beta 1 unless you installed a 5.0 Beta version (to be on Beta channel) and is still using it instead of Firefox 5.0 release.

To stay on release channel just download and install the Firefox 5.0

avg safe-search is maintained by AVG and grease monkey is a third-party extension like the majority of those hosted on addons.mozilla.org

I am on Beta 5.07. Firefox is sending me a SECURITY update for Firefox 6 Beta, but during installation, I am warn that my anti-virus and my script manager won't be compatible. Like I'm going to update knowing I won't have the security setting that I have chosen for years.

Unless you want to Test the 6.0 Beta 1 then just install Firefox 5.0 to stay on the Release channel and only get release update prompts whether security or next Major release.

Actually that prompt is not really a security update per say but a update option to try the 6.0 beta 1 since you are on the Beta channel due to still using the 5.0 Beta 7 as you said.

Actually there is a recent version of GreaseMoneky that will work, however not sure about the AVG extension at moment.

Thanks. Well, I updated to 6.0 beta since my script manager is now compatible. But mostly, because I'm fed up of being ask every few hours to update...especially when firefox is warning me that my security might not be up to date...How can they be more secure without my safe-search? (by the way, it's still not working). Why pester users to update if it's just a test? That's what I really like to know. Not enough volunteers?

If you don't want to be pestered by Beta update notification messages, get rid of the beta version and install a Release version; as James already recommended.

As far as AVG Safe Search, check with AVG for the availability of an update for Firefox 6.0. I surely hope that AVG is prepared for Firefox 6.0 being released next Tuesday.