There is a version of Firefox named Nightly, but most people do not have it installed. Currently:

• Firefox 24 (released)
• Beta (future Firefox 25, subject to some changes)
• Aurora (future Firefox 26, subject to many changes)
• Nightly (rapidly evolving future Firefox)

I can't answer your detailed questions about Nightly, however, please make a backup of your profile before removing it, if you remove it, as some users have reported losing all their personal data when doing that.

• Back up and restore information in Firefox profiles
• Profiles - Where Firefox stores your bookmarks, passwords and other user data

The Nightly (a1) and Aurora (a2) builds update each day, so when using such an early developer version you will see an update each day (sometimes more that once per day in case of a respin). Those alpha versions are more meant for experienced testers because you can encounter stability and regression issues at any time. The beta version (currently Firefox 25) can be useful for testing websites to see if they will get broken in the next Firefox release and file a bug report if necessary.

Note that you should always install such a development version alongside of a stable release and use a separate profile folder for each version.

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile

• http://www.mozilla.org/en-US/firefox/channel/

Thank you.