It is not reasonable for a user-freedom-oriented browser like Firefox to require you to install a separate version to install unsigned extensions. Hiding it in about:config behind a "this is dangerous, are you sure" warning would be the right thing to do here.
I think it's fairly reasonable for a user-freedom-oriented browser that also markets itself as a safe and fast alternative to Google Chrome. This exists to some extend to protect users who will click on any button as long as it makes them able to do what they want no matter how dangerous (which is, for example, why HSTS doesn't have a "Add Exception" button).
Mozilla focuses on maximizing the user's freedom to browser the web without being hindered by harmful addons.
And at the end of the day, Firefox is a Mozilla brand, using the Firefox browser associated with their Brand means to some extend that Mozilla will want to ensure that the average user has a certain experience with that brand. The average user is perfectly fine not installing unsigned addons, which is arguably something the more advanced and above average user might want, who has the full freedom to use a edition of the browser that is explicitly marketed to them, no?
