Unfortunately upstream has removed the API that makes it possible to add external spellcheckers, and the upstream selected library for spellchecking is unable to properly support a language with a structure of Finnish.
Currently there is no other option than to live without spell-checking support in Mozilla products, and eg copy-paste to and back from a text editor for spell checking. Or use a full Voikko library compiled into Javascript with dependencies like at https://www.puimula.org/htp/testing/js-libvoikko/js-libvoikko-demo.html -Timo
