Out of curiosity, under what circumstances would you consider distributing an extension bundle to be leaking its code? Unless I'm misunderstanding, isn't this the same file you'll be distributing to your users? At first bluff it seems similar to worrying about leaking your website's frontend (I've got news for you...).
If it's for an entire company, then it's easy enough to compile your own copy of firefox that accepts extensions signed with the company signature rather than mozilla.
Respectfully disagree - having to rebuild each time patches come out, on multiple OSes and versions, which have a patch to allow unsigned extensions is a massively more time expensive than developing a browser extension, and requires extra knowledge on the behalf of the persons responsible