Firefox now offers four channels of updates:
The nightly update channel is updated everyday, and the browser can go from being completely solid on e day to crashing on startup on another. This channel also brings the latest changes everyday. The releases are not testes and can damage your profile.
This is a completely new channel of updates for Firefox. It receives minimal testing and will generally offer the latest relatively stable Firefox experience in the latest version.
Beta versions of Firefox will be released from time to time in order to let a wide number of users test functionality that is coming in future versions of Firefox. Firefox betas are testes, but might have bugs that won’t be there in the release version of the browser.
This is the good old channel in which new stable releases such as Firefox 5, 6, 7 and beyond will occur. You will get an update only if it fixes an existing security or performance bug.
The nightly channel isn’t listed here, because it is only useful for a minority of users who are developing Firefox, or add-ons for it, and it is likely they already know where it is. If you’re still curious, go to http://nightly.mozilla.org/.