Sometimes the Firefox/Mozilla process gets hung up and needs to be restarted.
To do this, at a terminal prompt, type:
pkill -u your_username firefox
pkill -u your_username mozilla
Substitute your_username with your username. You can verify that the process is no longer running, by typing:
pgrep -u your_username firefox
pgrep -u your_username mozilla
Then try running Firefox again.
If this still doesn't work, then type:
pkill -9 -u your_username firefox.
pkill -9 -u your_username mozilla
Firefox/Mozilla should work this time.