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.
Firefox does this automatically to prevent crashing. There’s no real reason to unload tabs manually. If your operating system or Firefox needs more memory, then it will unload the tabs automatically. Unused ram is wasted ram. Don’t be scared by ram usage going up, it gets freed on demand.
Waaaaait, you can put emojis into your username?!
No, only into your display name.
There are reasons, and there are addons that allow you to unload tabs via their right click menu.
For me it is a way to keep tabs in a window for organization without them using cpu. In some sense it’s like replacing tabs with bookmarks that integrate into the browser like tabs.