Sure, if you're willing to forgo the updates and have to manually install each time, or have the updates convert you to the branded version:

https://blog.mozilla.org/addons/2016/07/29/extension-signing...

(Good luck compiling it yourself.)

I compile a unbranded version of Firefox for each released version on my private repository for AUR packages, automated and without the need of intervention at all.

I'm currently running the branded release but I could pick unbranded, beta, dev or nightly editions of Firefox if I wanted. It's not hard to automate and especially with the unbranded version you have permission to distribute the binary that results, a permission you don't have with the branded edition.

You can also use the Nightly or Developer Edition of Firefox as well.

Yes, you could. What about the person Mozilla claims to want to get into coding? They have to choose between getting the regular updates vs being able to tweak their addons.

And do you plan to set that all up in the middle of your workday when a forced update disables your extensions?

Again, you can also use the developer edition, which also allows unsigned addons and has automatic updates but also doesn't disable your extension. It's specifically meant for people getting into coding.

Plus, there is more ways than developing FF addons to get into coding that Mozilla wants to stimulate, developing the browser itself or websites is also an alternate goal you can join in addition to trying out making your own addons.

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?