Pros: If you use Firefox and like to live on the edge, then FireFix is for you! FireFix will automatically download and install the latest development copy of Firefox, but not before it creates a backup copy of your previous Firefox installation, just in case.
Cons: FireFix makes living on the edge easy, but nightly builds are not for everyone. Sure, some nightly builds offer new features and new bug fixes, but others offer plenty of new (and sometimes critical) bugs. This is, of course, to be expected. Bugs are part of the development process. If you want to live on the edge, you must be willing to accept the consequences.