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. If you run two Mozilla products that use the standard profile system and the same profile, it's possible, but not likely, that you will corrupt the profile if you make extensive configuration changes. You could lose your bookmarks and other content.

You can install two separate versions of Firefox and alternate between them. By default, you can't run them in parallel on Windows. If you start Firefox version A while Firefox version B is already running, all you get is another version B browser window.
However, there's no problem of this sort on Unix/Linux.

Solution
1. To put this hack in place, first grab a free Windows executable editor, such as Resource Hacker or XN Resource Editor.
2. Next, locate the Firefox binary firefox.exe in the install area.
3. Open it with your editor and drill down to the 102 and 103 entries in the StringTable section.
4. Change these values to an alternate value, such as Myfirefox.
5. Choose Compile Script and select File -> Save.
6. Now you're ready to start up the modified Firefox. Just run it as normal.