FF's biggest problem up to now has been memory leaks in its previous versions.
The big problem I see now is that it is producing new versions faster, much faster, than the developers of the add-ons can keep up with. In its race to keep up with IE, Chrome and Opera FF is losing fans who feel left behind because they can't use their favorite add-ons to make their browsing easier and more enjoyable.
I will continue to use V 3.6.* until they discontinue security upgrades to that version.
Really, I haven’t had any problems with add ons and I’m running version 7. (5th beta)
My add-ons are
adblock plus
beef taco
noscript
it also runs greasemonkey ok
My recommendation is to switch to version 7 when it comes out on Sept 27th.