One possible workaround would be to provide a way to add keys from which you would accept extensions. Then as part of the extension development process, you could sign your extension with your key when you are ready to install it.
If the issue Mozilla is really concerned about is security, this should be an acceptable solution.
No, indeed I do hope that security is the primary thing they are concerned about.
But it is possible they have other motivations (like control). I hope that is not the case, but I think it would be foolish to not even consider the possibility.
One possible workaround would be to provide a way to add keys from which you would accept extensions. Then as part of the extension development process, you could sign your extension with your key when you are ready to install it.
If the issue Mozilla is really concerned about is security, this should be an acceptable solution.