Got started learning XUL and figuring out how to write extensions for Firefox 1.5 and Komodo today. After a little bit of bang-head-on-monitor debugging, I've got a simple "hello, world" sort of thing happening, all nicely bundled into an XPI package.
The final product is going to be a front-end for a test framework I'm writing at work. Because the test framework is largely in Tcl, I originally envisioned a Tk front-end, but most of our developers don't have it installed. All of them have Komodo or Firefox installed, though.
My first impression is that things seem to have become much simpler as of Firefox 1.5, and I like that. Internationalisation is easy (so far -- I'm only handling one locale right now), and things seem to make fairly good sense so far. That being said, I haven't really done much with it yet, but time will tell.