(disclosure: I am a Mozilla employee but not commenting in any official capacity)
"Give me control over what code I run on my computer" (meaning "provide a switch to disable the requirement that extensions be signed") keeps coming up over and over. And perhaps it hasn't been clearly stated but the problem is this: if there's a switch that a user can flip, the browser has to record the state of that switch somewhere (presumably on disk). If such a switch becomes available, we'll quickly be flooded with malware that flips that switch without users' consent. At that point, there's no way to tell the difference between savvy users making an informed choice to enable unsigned extensions and malware doing it behind their backs. The browser can do various things to obscure the way that setting is stored, but ultimately any method the browser uses to read and write the state of that switch is something that other software can easily mimic.
This is not a theoretical concern, a modern web browser target is an irresistible target for all sorts of get-rich-quick scammers -- if you don't experience this day-to-day its due in no small part to the fact that browser vendors among others are constantly working to keep the bad guys at bay. But make no mistake: the bad guys are out there and they quickly find and exploit any opportunities that are available to them.
So as to the problem of how to let users disable signing but ensure that they have made a conscious decision to do so, there is a stark tradeoff here: giving the most savvy users that switch necessarily makes other users less safe. The solution that Firefox has opted for here is to handle this tradeoff differently on different channels. The release channel (aka the stable channel, or the thing you get by default when you download Firefox) is intended for a very wide audience, and so it handles this tradeoff by favoring safety for all users regardless of their level of technical knowledge. The developer edition and nightly channels are intended for more technically savvy users and they handle this tradeoff differently; specifically they do provide a switch for disabling extension signing.
If there are other (practical and effective) ways to solve this problem of determining true user intent, I (and I'm sure many many others) would be very interested in hearing about them. In the mean time, using the mass-market versus developer-focused channels as a signal for users' preferences on the risk-configurability continuum seems like a reasonable way to handle this.