Hi, Did you find the info you needed?
You probably missed this one. IMO it should get you started. Since a dictionary only needs the manifest file and the .aff and .dic file it should be relative straight forward to create the add-on
The tutorial about how you can create your first extension Your first extension - Mozilla | MDN also shows a way to locally test your stuff.
I didn’t create a dictionary yet, so I don’t exactly know if that works for them too. But I know it works for other add-ons.