Nooooooooooooooooooooooooo! XUL is really cool. Still one of my favorite GUI kits. It is like electron (but actually native!) and
had a good reason to be slow (spidermonkey wasn't super fast) :(
XPCOM kinda sucked but it is still better than MS COM
It's not that XUL was bad, exactly. But priorities have changed to security, battery life, and multi-threaded performance. The weight of XUL cruft that has built up over time was holding the project back.
How was XPCOM better than MS COM? Feature set? Nope. Tooling? Nope. Language support? Nope. Ecosystem? Nope. OLE components? Nope. Distributed COM? Nope. Doesn't trigger knee-jerk anti-Microsoft reactionaries even though it's a shallow clone of Microsoft technology? Check.
XPCOM kinda sucked but it is still better than MS COM