Rated 4 out of 5 stars
Using it with Firefox 3.6.25 "Legacy", since they are continuing to provide support for that version for the time being.
However, you can run multiple versions of Firefox on the same desktop OS if you know what to do and how to do it in Windows and Linux, the latter which you can ALSO run multiple versions at the same time as well, including "beta" or "alpha" versions.
Continue shoving out new versions faster than the developers of extensions and business/professionals can keep up, at your market share and user-base peril, Mozilla!!