And it is already marked compatible with Firefox 5, as well as Firefox 6 (currently in testing on the Aurora/alpha channel) and Firefox 7 (currently in development on the Nightly channel):
Alright, took the plunge. I never liked tree style tabs much in the past, but I've tweaked it abit here in Firefox 5, also got FireGestures & Firebug updated too. Good stuff, we'll see if I stick with it.