How to run multiple instances of Firefox?

My Firefox ESR installation has just updated itself to Firefox 68.2.0esr, from the previous version, and now I no longer seem to be able to run multiple instances of Firefox (using different profiles) the way I used to be able to before.

I could start the first Firefox instance from the dock, having previously set it up to ask me which profile to use when it started, and then, when I needed to start an alternative profile, I could do so via the terminal with:

open -n -a 'firefox'

However, this now just seems to open a new window of the already running Firefox, rather than a new instance (which the -n option is supposed to (and used to) start).

Is it still possible to run multiple instances/profiles of Firefox somehow?

Assuming you already have the proper profiles setup, try:

open -n -a /Applications/Firefox.app --args -no-remote

Mozilla seems to have recently unified behavior of the no-remote argument across all platforms.