This is an extension that adds a button to gnome panel which when clicked launches Firefox Profile Manager.
'firefox -no-remote -profilemanager' is the command that is actually executed.
Note: This extension isn't maintained any more.
git clone https://github.com/xtranophilist/gnome-shell-extension-firefox-profiles.git
mv gnome-shell-extension-firefox-profiles/ firefox-profiles@xtranophilist</code>
Alt+F2 to open run command prompt
Firefox Profiles Chooser Extension from Extensions from