I managed to install a downloaded dictionary but not in the way others have advised because there was no gog on the add on page as many others have stated. Therefore, I tried all the other options such as extensions and languages and hey presto there is the gog to install your download.
Worked OK once I worked out how to install. In Thunderbird. Download the file and then from the menu on the toolbar, select Tools, then Add-ons. In the centre window at the top is a gear symbol, select this and choose install from file.
Tried for 2 hours to get this add on to work. Most reviews are similar but the odd one which has been successful mention a 'gear' symbol which I don't have. Funny how spell checker works in this box. Perseverance paid off. *****
No instructions or help on installing this. The dictionary option in TBird is greyed out and the only thing left is 'Download more dictionaries' which brings me here. If you want this to be a truly viable email client then ridiculous situations like this must be avoided. I'm not allowed to give no stars, so one, undeserved star is what you get.