Do you have any evidence this is actually true? XUL is still there, the firefox UI is still written in it and it will be for quite some time to come. I find it hard to beleive disallowing XUL extensions have significantly sped anything up.
Disallowing XUL extensions made it possible to make many underlying changes that otherwise would break a lot of them. There are plenty of long-lasting tickets on Bugzilla being finally tackled in Firefox 57.