If you see a message “Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.”
This means Firefox is already running, possibly in another Ubuntu session that hasn’t been logged out.
Run the following command in terminal to kill the Firefox process.
killall firefoxFirefox should now run.
Let me know if this helped. Follow me on Twitter, Facebook and YouTube, or 🍊 buy me a smoothie.