Located this: https://addons.mozilla.org/en-us/thunderbird/addon/finnish-spellchecker/?src=hp-dl-upandcoming

Thanks, but that doesn't change the situation. I can't choose Finnish as default Language in Tools > Options > Composition > Spelling tab.

After downloading the addon, did you install it in Thunderbird? Please confirm.

How to Install in Thunderbird Download and save the file to your hard disk. In Thunderbird, Tools > Add-ons OR Menu icon > Addons Clickon gear wheel icon and select 'Install Add-on From File' locate the downloaded add-on and click on Open.

https://github.com/voikko/mozvoikko/wiki Sorry, do not know if the above link has info as I do not speak Finnish.

I tried the installation and noticed that it does not install into the 'Dictionaries' section, but into the 'Extensions'.

This means I can only select the Finnish spellcheck in the Write window. Write some text. Click on 'Spelling' I can select Finnish from the Language drop down option.

not a perfect solution, but at least it is available.

Yes, I did install it. I can see Finnish spellchecker 2.0.1.1-signed is installed.

On the addons-page there was link to bug https://bugzilla.mozilla.org/show_bug.cgi?id=522047

Yes, I can select Finnish from "Spelling", but I can't choose Finnish as default. Older versions of Thunderbird defaulted the last used language, so for me Finnish was the default.

On page https://github.com/voikko/mozvoikko/wiki this is a known issue :(