> ...whose functionality has been entirely superseded by HTML at this point...
This is just not true. Not all functionality enabled by XUL can be executed with Web Extensions. Perhaps this will be so in the future.
> ...and that's why care is being taken to make sure the important extensions continue to work...
Who decides what extensions are "important" and therefore worthy of working in this new "web extensions world"? I guess my point is that if there were true parity between web extensions and XUL there would be no need for concern. However, that is not the case and we know many extensions cannot be recreated. Therein lies the problem. Many users will be negatively affected by this change and I am still trying to understand the benefit. For example, why are Web Extensions mutually exclusive to XUL extensions? I get that supporting both is inelegant, but it at least reflects the current reality of Firefox usage.