Beta is a testing ground for new features that will eventually make their way to the release version of Firefox. You can install it (though I strongly suggest you uninstall glary, it's poor poor quality, as evidenced by telling you to install a beta product when you aren't already on the beta channel). Its up to you on if you want to use it, but expect a bit less stability, but faster access to new features on beta

You asked your question using the current Release version (29.0.1) In addition to Tyler's information.

In order of development - some new features may come and go from channel to channel.

• Nightly - Under heavy development. Least stable/secure. First tests of new changes/features; some changes/features introduced in Nightly may be removed before Release and other versions. Only for testing. Should only be used by very experienced users/testers.
• Aurora - Still under development. More stable/secure than Nightly. Some bugs may still be present that need resolution. Should be used by experienced users only who can post/report reproducible problems and work around issues.
• Beta - Final development stage before Release. Usually good stability/security. Major bugs resolved. Working out final bugs. Preview of what Release version will most likely contain in the way of changes/features, though some changes/features can still disappear before Release.
• Release - Final channel released to public for everyday use.