Firefox nightly, which will end up as FF57, uses less memory than Chrome and is faster. (Windows, x64). One of the reasons this was possible was disabling the old extension system. Does it hurt? Sure. Was it needed? Yes.
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.
I don't use extension any extension but i use Firefox on Desktop and Mobile. From end user wait time etc i dont see any difference between firefox and chrome. For some reason firefox is seem to be faster than chrome on mobile. I use firefox focus mostly on mobile. It would be really sad to see firefox go away as that is only one which is not tied to OS or tied to provider of webmail etc. I see firefox as a product from browser company.