I’d say fairness also requires mentioning how the browser has been able to improve because of killing off XUL extensions; technologically, they
were holding things back. Firefox is considerably faster than would have been possible without breaking a great many XUL extensions anyway. And more secure, if you care about that kind of thing, which normal people probably do or should, but frankly it’s the performance I care more about. So this is a “yeah, it’s a pity, but there
were good reasons and you
have benefited from it, even as you suffered” kind of thing.
Also how much more reliable long-term extension/browser compatibility has improved: I’ve used Firefox Nightly as my daily driver for about ten of the last twelve years, and until 2017 I’d spot at least one or two breakages each year, mostly fairly minor, but the occasional major (a couple of which accounted for maybe six months of going back to stable—and the lead-up to the killing of XUL extensions was another few months on stable because not all that I wanted was ready on WebExtensions yet). The extensions were typically patched before the change hit stable Firefox, so normal users wouldn’t notice most. But since WebExtensions, I don’t recall a single breakage. I acknowledge that the biggest breakages were in functionality that cannot be replicated any more, like Pentadactyl (and I ended up not even trying to replace it), but still, the minor and subtle breakages are just gone.