[karyon]

For a non-mainstream-feature I thought that rather easy. Open about:profiles, click "create new profile", enter a name, and then use the "Launch profile in new browser" buttons on that page to use that profile.

I have one for work and one for non-work and it's great!

[arendtio]

In addition, you might want to add an entry to your system menu or place shortcut somewhere to start a new instance:

  /usr/bin/firefox -P my-profile --no-remote

[unqueued]

If you want more fine grained control, check out https://github.com/dyne/tinfoil

I actually have some scripts for managing not just profiles but extensions. Hopefully I can get it cleaned up in time to put on Github.