- Go to your language folder of firefox. For me it is usr/lib/firefox/dictionaries.
- Note if it is a shortcut, for me it shortcutted to ../../share/hunspell (/usr/share/hunspell)
- Backup the folder somewhere (copy paste it)
- Go to that folder and DELETE everything other than the language you want. (for me I kept only en_US*)
This obviously only works if you don't want the other languages. After months of trying to solve this bug, I couldn't come up with any other way. Hopefully Mozilla fixes this bug soon...
Here's the command lines
user@user-1015E:/usr/share/hunspell$ sudo rm en_A*
user@user-1015E:/usr/share/hunspell$ sudo rm en_C*
user@user-1015E:/usr/share/hunspell$ sudo rm en_Z*
user@user-1015E:/usr/share/hunspell$ sudo rm en_G*