I assume you are using Firefox 52 ESR in that case the solution you need is given below. If you are using another version please provide me with more details on your firefox version and Operating system so I can look into it.

Since Firefox will only support webextensions from version 57 onwards DuoKeyboard has upgraded to this. In Firefox 52 ESR not all features are enabled by default yet. Please enter "about:config" in the adres bar and set "webextensions.storage.sync.enabled" to be able to use DuoKeyboard.