Earlier quoted context omitted.
It's not entirely unreasonable. We use software because it's useful. If you make it so secure that it is no longer useful, what have you got left? In the case of Firefox extensions, I'm aware of the risks of the old model, but the fact is that the only observable difference it has made to me so far is that when I accidentally hit back on a page with an unsubmitted form the other day I lost about 20 minutes' worth of…
The idea that the XUL extension model is necessary for a browser to be useful is obviously false. The majority of Web users use browsers that do not have a XUL extension model. (And, of course, I don't believe your desired functionality is incompatible with Web Extensions anyhow.)
I used to have a useful feature that worked.
Now I don't.
That was the practical result of the change we're talking about for this user, regardless of any theoretical benefits elsewhere or any theoretical ability to provide equivalent functionality within the new architecture.
Other people are welcome to use other browsers, but I was using Firefox, and a big reason I was still using Firefox despite various other changes I didn't particularly want was the range of useful extensions I could choose from.