MultiFirefox, originally developed in 2006, is a small launcher utility that allows you to run multiple versions of Firefox side-by-side. It helps you set up multiple profiles (one or more for each version of Firefox), remembers your last launch preferences for easy launching, has auto-updating built in for when we make improvements, and is 100% open source, MIT licensed.
It is written natively in Objective-C, but was originally written in Python and wxPython. Posted below are links to all of the pertinent downloads.
MultiFirefox has been downloaded hundreds of thousands of times since its inception, and discussed on Lifehacker (in 2008!!) and made their top downloads, was a staff pick on Apple’s downloads site, and was (and still is) used by some of the Firefox development team at Mozilla.
Original Python Release
The original source code for MultiFirefox 1.0 was released for Python and wxWidgets/wxPython. Left here for posterity only.