It used to be the case that I pored over Mozilla (and later Firefox) release notes. But I stopped sometime in 2004, around the point when I accepted that XUL development was too hard to be doing in my spare time and yes, Firefox was clearly better than IE at just about everything.
This means that I now miss all the cool announcements
http://webkit.org/specs/CSSVisualEffects/CSSTransforms.html
http://dev.w3.org/2006/webapi/selectors-api/#queryselector
http://www.dustindiaz.com/firefox-31-brings-good-stuff/
Actually, I think XUL development is getting off the ground, too. With stuff like Spicebird, Songbird, Instantbird and Flock, and Thunderbird moving off into its own company, people are actually using XULRunner, so it’s getting better and better.
I think it’s been slow going, and reasonably painful to get even to this state.
Also, I think it’s easy to see from the pace of change for those tools and long-lived beta status that while things are getting better XUL has not been the massive success some of us anticipated in the early noughties.