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.
Right now?
So a fair few. Although I can probably close the UPS live chat tab because I’m getting nowhere with these idiots.
This reads like the dystopia in which every piece of software got replaces by a proprietary web application by some evil mega corp. What you need is at least a mail client and a word processor.