You can edit your shortcut or .desktop file to open "firefox -P" instead of "firefox". Each time you click it, it will ask which profile you want the new window to be in.
Had I known this earlier... I had the same reaction as the person you replied to, I hadn't thought of using profiles. I knew something called profiles existed, but I thought it was not in use anymore (at least not properly supported). I never connected the dots.
Still sounds like a pain to manage though, compared to this new feature.
Yeah it's not a great user interface, and it's definitely buried. Aside from the Developer Edition which uses a different profile by default, it seems to be ignored by Firefox devs. Probably because they don't want to support all the people who get confused by the terrible UI!
Still sounds like a pain to manage though, compared to this new feature.