Why isn't my Firefox or Mozilla starting up?

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

or

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

or

pgrep -u your_username mozilla

Then try running Firefox again.
If this still doesn't work, then type:

pkill -9 -u your_username firefox.

or

pkill -9 -u your_username mozilla

Firefox/Mozilla should work this time.