Firefox containers[1] are new, but you could always do this with Firefox profiles. If you're on Linux or Mac, open a terminal and type:

    firefox -ProfileManager -no-remote &

(I alias it to `ff`.)

Create different profiles[2] for each separate browser you need, and then make shortcuts to launch them.[3] Each Firefox profile will work like a completely separate browser.

[1] https://addons.mozilla.org/en-US/firefox/addon/multi-account...

[2] https://support.mozilla.org/en-US/kb/profile-manager-create-...

[3] https://developer.mozilla.org/en-US/docs/Mozilla/Command_Lin...