firefox-gtk2 should not own that file.
pacman -Qo /usr/lib/firefox/distribution/distribution.ini
/usr/lib/firefox/distribution/distribution.ini is owned by manjaro-browser-settings 20161221-1
Check the firefox-gtk2 filelist
pacman -Ql firefox-gtk2
and if it's in the list, remove
firefox-gtk2(AUR). If you must have it you can modify the package build and re-install.
You might try
sudo pacman -Sy manjaro-browser-settings
to reinstall it and that might fix the database.