The feature is called Tab Unloading, and weirdly enough they made it not easy to access despite its usefulness.
You basically have to type about:unloads in the address bar and hit enter. If you then click on “Unload”, it will put the least used tabs to sleep. If you keep clicking that button until it’s greyed out, you’ll have unloaded all your tabs from memory.
This feature is handy if you want to temporarily switch to something that is memory hungry without having to close your 100 tabs.
Nope! Not happening or at least not soon enough. Neither on macOS or Linux (can’t speak for the stupid platform).
Firefox will happily keep tabs open, even if macOS reports major memory pressure or Linux needs to invoke the OOM killer because it’s Gigabytes into swap.
Not to speak of what happens before memory pressure is reached; Firefox will also happily use all of your memory even if you’d rather have it free for something else you’re going to do next.