I started a thread entitled Mozilla tips and tricks over at SitePoint Forums in hope of getting people to post their favorite extensions and tips, as well as possibly promote Mozilla (I'm a FireBird addict).

Other than those I'd found, a member posted a link to http://extensionroom.mozdev.org/ which seems to have a very wide collection of links to extensions. Check it out.

Oh yes if you're using Mozilla or FireBird now, you can see XUL in action just by right-clicking the toolbar (where the 'forward', 'backward', 'refresh' buttons are) and choose 'Customize...'.