Yesterday's Ubiquity lab reminded me that I need to tell you how to deal with having multiple browsers running at the same time. By default, only a single version of Firefox can run at a time. This is especially problematic when you're also using ChatZilla as your IRC client (which I recommend as you begin learning to use IRC).
Another tip is to learn how to start more than one version of the browser at the same time. On Windows, with one version of Firefox already running, navigate to your nightly or custom build directory and run the browser this way:
C:\Program Files\Minefield>firefox.exe -no-remote -profilemanager
What this does is start-up a second version of Firefox instead of opening a new window for the current one (that's what -no-remote does), and also start the Profile Manager, which lets you select or create a new profile, different from the one being used by the other browser (probably the 'default' profile). It's always a good idea to not use your default profile when testing experimental versions of the browser, since new versions can change or sometimes corrupt your exisitng profile.