I've found out that the simplest way to run multiple versions of Firefox
side-by-side without having to engage in Profile Manager acrobatics/silliness is
to use Firefox Portable and just set
FirefoxPortable.ini can be found inside the
but the trick is that you should copy it to the main directory
Other/Source/../.. or, in other words,
that's what you named your main directory) otherwise it won't work.
I discovered this great tip thanks to this page: