Update: Language packs are exempt from the signing requirement, however the add-on does not have its type set (I'm not even sure if add-ons had types when I created it). The fine folks at Mozilla have detected the problem and will automatically set the type correctly soon, so the problem should go away. If not, I will upload a new version with the correct type.

Received an email saying there would be no compatibility issues with v43. Was not aware of this new signing requirement. Will investigate and sort it out next week when I have some free time.