HOW TO: Run Two Separate Versions of Firefox Simultaneously
Unless you’re extremely technical, Internet Explorer is a one-shot affair: either you install it once or
you don’t install it at all. Firefox is not like that; you can install many versions and related products and pick between them as you see fit. Mozilla-based browsers do not yet fully support profile sharing between products. Firefox and Thunderbird are a partial exception, since they overlap only in a few profile files, such as the certificate database.
How To Run Two Separate Versions of Firefox Simultaneously