As far as I can tell various languages (en-us, fr) come with a dictionary installed by default. Unfortunately this is not the case for the German version. I would assume this is the case because there are many Germany dictionaries. If we can detect the location it would be great to use the matching one, that is Austrian, German or Swiss. However, having the default German dictionary installed, would probably be better than having none. (https://addons.mozilla.org/en-US/firefox/addon/german-dictionary/) (Tested on Firefox 43.0.4 64bit (en-us, fr, de) on Windows 10)

This is due to a incompatible license. For German ispell98 is used which uses the GPL. French and en-US have dictionaries derived from libraries / dictionaries with different licenses.

This is a WONTFIX as long as there is no good-quality German dictionary with MPL-compatible license (and in a HunSpell format). Feel free to start a project for a more permissively licensed dictionary, we'd be happy to ship it by default once it has enough words and we can verify it's working decently.