These are for developer testing only and not recommended for inexperienced users.

Here's an extract from Mozilla's "Developer Central" web page;

"We make nightly builds for testing only. We write code and post the results right away so people like you can join our testing process and report bugs. You will find bugs, and lots of them. Mozilla might crash on startup. It might delete all your files and cause your computer to burst into flames. Don't bother downloading nightly builds if you're unwilling to put up with problems."

While ... cause your computer to burst into flames may be stating the difficulties a little too far, unless you are prepared to put up with major system failures and possible re-installs, then it may be better to walk away and wait for the full versions.

Mark