I've had that same issue in the past, i.e. not knowing about the loss of a prized extension until after I upgraded to a new Firefox version. Unfortunately, there is no easy "undo" method. You'll have to reinstall older version of Firefox, reinstall extensions, copy over bookmarks, passwords, etc. I suggest you look at password exporter, copy your bookmarks, FEBE for extensions.

In the future, install upgrades only after creating a new profile to work with the newer Firefox version. You can Google for the instructions to accomplish this. See what works and does not work before you decide whether to take the step to the latest Firefox version.