Re: I have Firefox version 18. If I download and install Firefox Beta, will it be a separate browser or overwrite my installed version?
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.