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 AllowMultipleInstances=true
in FirefoxPortable.ini.
FirefoxPortable.ini can be found inside the Other/Source directory,
but the trick is that you should copy it to the main directory
(e.g. Other/Source/../.. or, in other words, FirefoxPortable/ if
that's what you named your main directory) otherwise it won't work.
I discovered this great tip thanks to this page:
http://www.makeuseof.com/answers/run-multiple-versions-portable-firefox-parallel/