I'm not sure what "multi-process" is either, but honestly i haven't noticed any weird behavior by having it disabled. My guess would be say you have Pandora streaming in one tab, and you open a new tab that wants to play audio/video, i think it'd kill the stream in the other tab, but that's just a guess. It has no effect on having 15 tabs open on normal web pages in my experience.
Sadly firefox 56 will be the last version to run on Lion. I tried with 57 and it bombed. 56 still feels and works the same as the last "legit" release (48 i believe) for Lion and we get a bit better security with it. Also we can still run current releases of TorBrowser and Seamonkey by editing the plist file, and Palemoon still supports down to 10.6 without any modifications.
As for Tor, i just used the tor browser bundle which is a mozilla/firefox based browser with tor built in. See here:
https://www.torproject.org/projects/torbrowser.html.en
I really haven't tried with much other software as all i really needed was an up to date browser, and i use google drive via the web page interface. I think maybe it worked with a newer Spotify version however. Your milage will vary, but even if 2 out of 10 apps you try work, it's still worth the few minutes it takes to experiment.
Cheers