I don't think anyone mentioned the command line option --new-instance firefox --new-instance
Probably you should use it with --ProfileManager firefox --new-instance --ProfileManager Alternatively you can enter about:profiles in the location bar and start a new one from there. If firefox is slow you could try --safe-mode and see if that is faster. If it is it could be some extension, or settings, or accumulated data which is slowing it down. Use the process of elimination. firefox --help
