To solve the Firefox language issue, I had to replace Firefox and the language packages.
For those that want Firefox in their own language, type these commands in a terminal:
locale | grep LANG=
This will show you what locale you have, e.g.: es_MX
The Firerfox language code then becomes: es-mx
Now you can update the cache, and reinstall firefox with the language package:
sudo apt-get update
sudo apt-get purge firefox*
sudo apt-get install firefox firefox-l10n-[my language code]
Replace [my language code] with your language code.
The above example would make: firefox-l10n-es-mx