It's not just me!! Many many questions about installing additional (spelling) dictionaries have been posed by the "community". Installing additional (spelling) dictionaries used to be straightforward. Downloading dictionaries is still not a problem, but getting Thunderbird to take them into account is another story. Would it help if you indicated where they should be placed once they are downloaded? In any event I eventually succeeded but I don't know why. Perhaps because I first downloaded the dictionaries to Firefox. It's a pity that something that worked well is now a problem . . .