Well, after some initial testing (basically installing FFX 3 on a VM before replacing the real one), I've moved to Firefox 3.0.
It's been not very painful, but I had to search for a few "3.0 betas" of some addons, plus I've lost one of my addons (not updated since 2006, so incompatible with 3.0).
I've updated my Installed Firefox Addons list to reflect what addons are not compatible (just 2), and I'm now searching for another "dark theme" for the browser, because "Vista Black" is incompatible too :(
Performance improvements are here as we've been told... having tons of not-trivial addons installed, memory consumption has been reduced to 50% of previous version (still at 88MB but hey, I've got an FTP client plus a web development environment inside the browser ;), and loading and rendering speed of pages is noticeable faster! I feel very happy of the new version.