I have both German and Dutch dictionaries downloaded, but firefox' spelling check just... doesn't see the dictionaries at all. (The dictionaries tab eventually links to the second screen, when you can see the pack is installed.) I can't seem to find anywhere else it might need to be activated in first, my computer's default language has also been either dutch or german so that shouldn't have any effect. It's not a restart firefox issue either - it has persisted for a while, i just keep forgetting lol.