Firefox's Multi-Account Containers changed the game for me back in 2018 or 2019, and is what got me to switch back to Firefox for 95% of my personal browser use. The "Always open {{domain}} in this container" and the ability to open a specific container with a keyboard shortcut seem like small things, but I can't imagine life without them anymore.

I never understood why it was set off to the side in an (official, first-party) extension, because I was always afraid they would one day stop supporting it. Now when I explain to people why I prefer Firefox, I don't have to explain "There's this extension, don't worry it's official, that allows me to..." I can just say "It has containers built-in. They're great. Try them."

Yep, same here

MAC + Temporary Containers extension is how web browsing should be handled by default imo

No website deserves permanence of any data in my browser unless I so choose

I haven't seen similar functionality in any of the new generation of browsers, nor any of the Firefox spinoffs as-yet