It will be separate, but will use the same bookmarks, history, etc. that Firefox release uses.

hello, it will overwrite your currently installed version - but your bookmarks, passwords, customizations etc. won't be affected by that.

edit: tyler are you sure about that - i think the beta version will install into the default firefox program folder & replace its contents?!

It may install in a separate program folder, but will still use the currently selected default profile, so make sure to do a custom install and install in another program folder.
Don't launch Firefox after the installation has finished, but use the Profile Manager to create a new profile and use a desktop shortcut to start each version with its own profile.
The beta version is supposed to work just like a release.
Note that the next beta version (probably the last one) is planned for Tuesday next week.

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox
• http://kb.mozillazine.org/Bypassing_the_Profile_Manager

Use the -no-remote command line switch to open another Firefox instance with its own profile and to run different Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile