Thank you, Igor! I tried pretty much all other add-ons, including the Mozilla-recommended one, and none work (anymore). Looking at their Github pages, they seem to have been abandoned, with lots of people having problems.

I didn't have many hopes for this one, since it also was last updated over 1 year ago. But, joy! It does work!

Works great. Add more dictionaries please. I would like to be able to choose what dictionaries to look into.