Other people have already answered your actual question, so I'll weigh in on my assumptions of your use case instead :).

I find that multi-account containers give me much of what I would otherwise gain from running in multiple profiles, in as much as cache, cookies and other state are separate, so I can be logged in to a different set of accounts in each container. Temporary Containers makes life ever easier -- a whole new empty state at the push of a button, and parallel non-shared state: it's much more usable than Incognito for web development and testing.

On the other hand, if you want a different set of extensions then multi-profile is the only way to go.

https://addons.mozilla.org/en-GB/firefox/addon/multi-account... https://addons.mozilla.org/en-GB/firefox/addon/temporary-con...

I think you can already do that?

Yeah, just start the profile manager (firefox --ProfileManager), create a new profile, and then run the "other instance" with --profile NAME. Sometimes --no-remote is also helpful, so clicking on some links will not open them in this other profile.

This can also be done from within the browser: navigate to about:profiles.

You can, if you run Firefox with -P and profile name. I run two instances most days.