"great" may be stretching it more than a little. They have indeed worked for a very long time (which is definitely great), but launching multiple firefoxes at once with different profiles is (has been? haven't tried for over a year) an absolute nightmare for anyone not launching it exclusively from the CLI with aliases. I actually made app wrappers just so I could click things instead of having a weird blend of "you can click that icon... but then you'll just get the default profile" + "CLI if you want [profile X]".