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.
Regarding pentadactyl, I'm using tridactyl on Firefox and essentially for all my use cases it behaves the same as pentadactyl. I realise that some of the advanced functionality of pentadactyl is not available (IIRC pentadactyl allowed you to essentially reprogram the browser), but I have to say I'm not really missing much.
In most areas (though certainly not all), the difference was generally ironed out well before the advent of WebExtensions—what’s come since then has been as often surpassing as catching up.
Chrome was definitely a wake-up call, and the Firefox 4 cycle in 2010 achieved a lot. I recall doing audio stuff with the new Audio Data API (sadly since discontinued in favour of the much-more-complex-generally-for-no-good-reason Web Audio API) with a sine waves stress test (adding random sine waves together until underrun occurs) in Nightly, and watching the number it could cope with increase, week after week, due to JIT engine improvements. It went from handling a dozen to handling a couple of hundred over the course of two or three months.
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.