Cool, yeah, noone can complain unless they can personally do it all better. Do you think that's workable?

I don't think it's trivial. The critical element here appears to be "who gets the final say" and not "this is to hard to code".

They manage to disable the "Enable" button for addons, and managed to consider this situation enough to provide a justification that (paraphrasing) "we do this when we don't want the add-on installed", which is harder to do, they've added extra tests, added complexities and done all the consideration. They've just chosen to remove the final say from the user and give it to themselves.

It seems consistent with their recent behaviour.

No, you can complain. I specifically object to “How hard is that?” which is an entire class in itself. Stuff often is inherently hard and when you don’t know internals of a project and don’t work on it, you may have no concept of how hard it is. Don’t pretend you do. Saying “How hard is that?” carries the notion that everyone working on that thing you don’t know is sloppy, malignant or stupid.

“I wish it would do that.” is a much more charitable way to phrase your complaint.

Are you saying you don't think that Mozilla have the capabilities; I'm not. My "how hard is that [for Mozilla]?" is specifically "I think they have the capabilities but chose differently, but if I'm mistaken and there is a technological bar to this then please correct me". That's why I didn't write "That's easy!", nor "How hard is that!", but used "how hard is that?" -- the implication is that's not hard for them to do so why did they chose to do it differently.

As it happens I've just had to flip "xpinstall.signatures.required" and it's working for me. So it seems "not at all hard [for them]" was the answer.

FWIW people have chipped in saying this specific issue was raised, so it's not that they hadn't conceived that such a situation could occur (indeed that's surely why the config above exists).