One thing to remember is that when the Mozilla team originally created XUL browser engines were a different beast. At the top you have JavaScript which is _significantly_ faster on many different levels. Combine that we current html features supported (and or optimized) by some WebKit ports such as hardware accelerated CSS transitions and you get a much a strong base to create applications on top of. Also writing large JavaScript applications were new back then. These days we have several years of lessons about what works and what doesn't. Lastly the tool situation is much better. Simply what you can get if you limit yourself to inspector is pretty crazy. Remember Firebug didn't even come out until 2006 if I recall. And if all that isn't good enough the HTMLRuntime is multi-process so you absolutely wont run into the contention for the JavaScript engine between the chrome and the pages. To see some animations in action checkout the above video links.

Disclaimer: I am part of the WebKit team at RIM these opinions are my own