How to fix the "Firefox already running" problem on Linux

After creating a script which included closing a firefox tab and hiding the window I realised I could not open firefox and just got a "Firefox is already running" error every time after running this script once.

I tried running the command
sudo killall firefox
As there may have been some hidden windows however this didn't work.

Then I tried reinstalling firefox
sudo apt-get install --reinstall firefox
This didn't seem to work either.

Note this command will remove all personal data from firefox, run at your own risk.

After some research I found this
sudo rm -fr /home/"USERNAME"/.mozilla
This worked and I could open firefox again however I don't believe its the best way of solving this problem.

I'm open to any better ways so post in the comments if you have any.