Managed to kinda solve it with AI help. Still working weird but acceptable. about:config -> layout.spellcheckDefault -> from 1 to 2 Surprised not to see such setting in basic options though.

Same problem. I have the check box checked. I have the language installed. I have the correct settings in about:config (have tried setting it to both 1 and 2).

It just doesn't work. Ended up installing an extention. Which is bloated and annoying, but at least it actually works.

In Chrome spell check works perfectly.

Same problem. I have the check box checked. I have the language installed. I have the correct settings in about:config (have tried setting it to both 1 and 2).

You say language which makes me think you installed a Language Pack and not a Dictionary.

On top of https://addons.mozilla.org/firefox/language-tools/ it gives a basic idea of what each are used for.

Installing a dictionary add-on will add a new language option to your spell-checker, which checks your spelling as you type in Firefox. Language packs change your browser’s interface language, including menu options and settings.

Only a few language locales of Firefox comes with a dictionary due to licensing reasons. https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

A Language Pack is useful for people who want one Firefox install but be able to change their interface between two or more languages and not just use the language locale you downloaded and installed.