Which version of Firefox have you got at present? Firefox 5 was a major revision (which resulted in quite a few add-ons becoming incompatible) but Firefox 6 represents very little change. Almost all add-ons which work under Firefox 5 will continue to work under Firefox 6.
Because add-ons are developed by independent programmers, it's not up to Mozilla to make each add-on compatible with the latest version of the browser. It's up to the individual programmers (if they're still interested in being involved with the Firefox project) to update their add-ons.
The most notable thing that I noticed (even with FF 5.5) was the fact I couldn't have Google toolbar, which in Add-ons' I had Spell-check, you-tube, translate, auto-fill etc...
But it doesn't work with FF6 either.