I am guessing your configuration files are corrupted.
I have had this happen on a few occasions and to be honest, easiest thing to do iss to move your entire home directory.
Boot to recovery mode , run a root shell
Code:
mv /home/your_user /home/your_user.bak
mkdir /home/your_user
chown your_user:your_user /home/your_user
then exit and at the recovery menu continue booting.
You will loose your configuration for your desktop, but should be easy to fix.
If you need data or other files from your old home, move it from the back up (your_user.bak) to the new home.
Once you are happy your have moved everything you need, you can delete the old (or back it up off your hard drive).
IMO this is easier then attempting to restore corrupt config files.
The one file you might want is /home/your_user_bak/.mozilla
Good luck.
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