Disagree, XUL was a dead end that either needed shooting behind the bike shed or it’d have taken Mozilla down with it inevitably. It froze their internal architecture to a design that didn’t care about multicore or modern security. Switching to a proper extension api (it didn’t matter if it was chromes or their own, only that they are willing to make their own decisions, like in manifest v3).

That said, I suspect the real death blow was when they killed servo, that project was their distant salvation, a chance to genuinely outcompete technologically and direct where browsers need to go next. I too hope I’m wrong and they can figure out a path forward, but they’ve shown little ambition from the top, so I’m not holding my breath.

Edit: you could argue that the solution to XUL should have been an upgrade to modern design rather than death, but that would have just been an expensive temporary reprieve, the world doesn’t stop changing, it was always going to be slow to correct to whatever direction they needed to go next (and meanwhile every extension dev would be screaming murder every time they killed some braindead api designed 20 years ago).