Install xul-ext-mozvoikko by entering the following commands in the terminal:
sudo apt update sudo apt install xul-ext-mozvoikko
Description:
Voikko spell-checker extension for Mozilla products
This extension provides a spell-checker front-end for Mozilla products (Firefox, Thunderbird, etc.) that utilizes the Voikko back-end to check the spelling of texts written in Finnish and other similar languages.
Homepage: http://voikko.puimula.org/
Version: 2.2-0.1ubuntu1
Section: universe/web