Not the parent, but Firefox Quantum while an advancement in some ways, also killed XUL Extensions and Mozilla moved Firefox entirely to WebExtensions without the APIs to fully support existing popular add-ons. NoScript for example is a shell of what it once was, every vim keybinding extension was pretty much cutoff, TabMix Plus discontinued development since many of its popular features weren't possible with the WebExtensions API and there still isn't a great tree-style tab extension.
Most likely the parent used one or more of these, as they were some of the extensions you could point to that Firefox had but Chrome never really did, and without them Firefox arguably doesn't have the same appeal.
There's still plenty of reasons to use Firefox over Chrome, but there are also plenty of users bitter about the loss of their previously working extensions.
> killed XUL Extensions and Mozilla moved Firefox entirely to WebExtensions without the APIs to fully support existing popular add-ons
this. I was never a tree view tab convert, but definitely miss noscript and vim bindings, plus things like the selenium UI.
XUL sucked in many ways, I tried writing extensions with it and I have no illusions there. But coming up with a reasonable upgrade strategy for a huge swath of popular extensions was something that should have been done before deprecating it. Rather mozilla basically told people that if webextensions didn't do what they need now, then just hope for the best sometime in the future, and in the meantime too bad, your extensions are gone.
But honestly this wasn't the only thing, just the most recent. It's the general attitude of willingness to ignore the actual use cases of their actual users for some theoretical appeal to a mass market of "average users" that they've yet to convert. I felt much the same way after the Aurelius release broke a bunch of ui, and any number of other breaking changes over the last few years.
This being HN, I'm operating under the assumption that most readers are developers.
I'd suggest that you read my colleague's comment at https://news.ycombinator.com/item?id=15696184 to understand just how much friction the legacy addon ecosystem was creating. It just wasn't sustainable.
You won't find me arguing against Mozilla's decision to drop XUL extensions, I'm just stating a fact, that it was one of the primary differentiating factors between Firefox and its competition, for better because of the varied high quality extensions that you simply would not find anywhere else and obviously for worse.
Which?