I have been playing with the latest iteration of Firefox i.e. Firefox 3.6 (RC1). It works great despite being RC1 and no bugs or crashes till now and all of my Firefox add-ons too work with it.
The newest addition to Firefox is Personas, sort of like the old themes Firefox had but better.
The Personas (skin or theme changer) option is at the bottom lower left hand of Firefox and all it needs is a click for Firefox to change its clothes. The add-on for Persona can be downloaded and installed from here. Very simple but effective and powerful for those who want to change the appearance of their browser every day or every so often. Personas and Jetpack between them are expected to replace the default Firefox add-ons and plug-ins as we know them.