Not that this needs another review, but this is one of Firefox's really killer features. It's like having separate browser profiles, but soooo much simpler to use.

I use one for each Client I do dev work for to keep the SSO logins separate. So, when I switch from one client to another, I don't have to specify my user every time on MS sites, etc. Every time I go to a website that uses SSO, if I'm in the right container tab, I'm logged in and ready to go. It's saved me so much time and headache.

Like, I have no clue why some of my dev coworkers are still on Chrome/Edge/Whatever Chromium clone. This feature alone is enough reason to come to FF. (To say nothing of the other great stuff, including the extremely important mission of "preventing 1 single company for-profit business from controlling the future of the browser. (i.e. even most alternatives use Chromium, which is "open source", but really controlled/developed by Google.)