Try closing Firefox, and then launch the Firefox Profile Manager by typing this into a terminal:
In the Profile Manager window, click `Create Profile`.
Click `Next` and enter a new name for a test profile. You can use any name. Doesn't matter. This is just a test.
Click `Finish` and continue to launch Firefox with that new profile.
See if Firefox works as expected.
If it does:
we know that your problem is in your Firefox settings
If it still does not work:
we can assume something is wrong with Firefox itself