They don't document it well but they will sign pretty much anything over the web so long as you're not distributing it on add-ons.mozilla.org (AMO). You don't have to use web-ext.

There isn't even a manual review gate for AMO releases after the first one (but it can happen after that fact).

I find Mozilla's policies friendlier than Google's but this is the first I've heard of "extension developer mode" - will have to look into that as my extension is cross browser.