Some quick useful tips to use the latest beta without losing all your addons. Download Firefox 3.1 Beta and just save it in a seperate folder so you can easily switch from 3.1 Beta to 3.0.* with no problems (NOTE: This probably will not work with Windows or Mac OSX versions since it will just over write the existing installation).
Enable all your addons: visit "about:config" right click and choose New -> Boolean and enter "extensions.checkCompatibility" and set it to false. This doesn't check your extensions to see if they are compatible and should work fine in most cases.
Check out the new Ctrl+Tab feature. Open 3 or more tabs and switch between with ctrl+tab and you'll see a new 3D visualization like you see in most operating systems with alt+tab.