Firefox2 and Firefox3 coexist under Windows (2)

Install Firefox3, select "Custom" for "Installation Type", so you can choose to install a new installation location. Although the default folder name is Mozilla Firefox 3 beta 2, it's best to redefine one yourself so that you don't have to make changes to upgrade the new version later.

At the end of the installation, do not select the "Run Firefox now" option. If it runs now, it uses the default configuration file, which is not what we want.

Right-click on Firefox 3's shortcut and select "Properties" in the target input box -no-remote -p firefox3. This step is to tell this shortcut to run using the configuration file Firefox3.

This way we can run Firefox2 and Firefox3 on the same system at the same time. Unfortunately, Firefox 3 basically does not support add-on on Firefox 2, including Firebug. Fortunately, there is already a Firebug 1.1 beta for Firefox 3.

This method comes from How to Run both Firefox 2 and Firefox 3 Together.

Just found MultiFireFox, this is a cross-platform program written in Python, interested to compile and try. A dedicated MultiFirefox DMG is available for Mac users. Thank you fireyy