I have the add on installed and it does not do anything while I am writing in google mail. It even does not come into the language list, while it says it is installed. Very strange and difficult to count on.

It simply does not do anything...

This add-on does not work in Firefox 69.0 64-bit running on Microsoft Windows 10/version 1903, build 18362.356.

Sometimes good, sometimes erroneous.

geweldig programma

G

slecht woordenboek

Does not work in Windows 10. After many years this is the reason for leaving Firefox and will use Chrome

IMPORTANT !!! Everyone. For 'Spelling Check', download 'Woordenboek' from this page https://addons.mozilla.org/en-US/firefox/language-tools/
This language pack cannot be used for spelling !
I was confused too.