Long story short: the way I understand it, Firefox add-ons need to specify which version of Firefox they're compatible with, in case there is version-specific code they require to function. Most add-ons, however, don't have that version-specific code, so Nightly Tester Tools (basically speaking) futzes with add-on version lists so (ideally) more recent versions of Firefox - i.e. this shiny new 2.0 release - can use these add-ons just fine. The one catch I've personally run into is that it seems Nightly Tester Tools needs to be installed on a v1.5.x of Firefox in order to be able to work with v2.0 (ironic, isn't it?). I stumbled across this because I recently wiped my MacBook Pro and reinstalled Mac OS X (I needed to start from scratch to get that new version of Vista running on my MBP). Along with this fresh system, I tried installing Nightly Tester Tools into a Firefox 2.0 beta, and it wouldn't budge. Turns out I had to degrade to 1.5.x, install Nightly Tester Tools, then bump up to 2.0, but YMMV.
I need to give a shout out to Download Squad, as it is they who first found this add-on which makes playing with Firefox betas so much sweeter.