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.
I would like the opposite though. When I used chrome, my WhatsApp web tab would load even if I didn’t open it, so I still got notifications. In Firefox I have to manually switch to that tab every time I open it.
I find pinning my WhatsApp tab sorts that out, it seems to autoload pinned tabs into memory on launch.
If this works, THANK YOU. It’s been annoying me for so long.
EDIT: it does work.