Want to use FF2/3 side by side? Check this. Although it describes the procedure for Mac/Linux, everything is almost the same for Windows. Just check this recommendations:
- Download the *.win32.zip from the night build.
- Unpack on a new directory, say \Program Files\Mozilla Firefox 3
- Create the new profile as described
- Create a shortcut to \Program Files\Mozilla Firefox 3\firefox.exe with options:
-P newprofile -no-remote