When I was reading this, and the author mentioned XUL.dll taking a long time to load I was assuming the blog would veer into a discussion about what can be done to refactor that code to make XUL.dll smaller. Can it be broken into separate DLLs, with the core portions loaded first? Is there a better design that could be used? But, no, their solution was to show the windows soonest. That's nice, I guess, but it didn't solve the problem. It just solved the user's perception of the problem. It actually increases the load time, so now you have an unusable window for a longer period of time. That's one of my pet peeves - don't give me an interface until you're ready for me to start clicking on things.
XUL is the name of Mozilla's old UI language [1]. They've been in the long process of moving UI pieces from XUL into HTML5 (such that the Browser's UI controls use the same renderer as the Browser's contents). One key piece of that was disabling the ability for extensions to use XUL a while back that caused a lot of churn in Firefox extensions.
The impression from that is that they want to entirely get rid of XUL.dll at some point in the future.
