every single time, Firebug and Greasemonkey stop working.
If you use a beta version of Firefox and want to use Firebug, the Firebug developers say you should use the beta version of Firebug. It would be nice if Firefox Beta automatically went out and fetched the beta version of Firebug instead of just saying the version you have is incompatible.
Greasemonkey is often more compatible than its authors let on. I'm using it with Firefox 8 Nightly and it's working fine, despite being marked as only compatible with Firefox 5. I bet it would work in your Firefox 6 Beta just fine.
There's also a competitor to Greasemonkey called Scriptish that is marked as compatible with Firefox 6 beta. I've heard good things about it but haven't tried switching yet.
track add-ons better and not refuse to load them just because they haven't yet been certified to work
That's the plan!
AMO-hosted extensions that use APIs that haven't changed are automatically assumed to work. So are extensions developed using the new SDK.
To do it safely for other extensions, we'll need to gather data from beta users (like you -- thanks!) to find out whether the extension still does its job, whether it causes crashes (crash-stats correlations), and whether it causes other widespread problems (telemetry correlations).
Future beta versions of Firefox will probably ask you whether you also want to beta-test extensions that might not be compatible. For now you have to set a hidden pref to do that.