Firefox won't start

If you get a message like "Firefox is already running, but is not responding. To use Firefox, you must first close the existing Firefox process, restart your device, or use a different profile" that means you have a lingering Firefox browser running on another machine.

If possible you should cleanly end that session before starting a new one. If that's not possible and you wish to leave the current session untouched then you can launch firefox from a terminal window like this:

firefox -P &

which will give you the option to create an alternative empty profile.

Otherwise, you can simply delete the lock file which firefox creates, from a terminal window run the command:

rm -f ~/.mozilla/firefox/*/.parentlock