Over at Peer Pressure there's a post on asking for an Eclipse-like IDE for XUL apps to be written on top of Firefox (or XULRunner to be technical). I agree with Alex's post, however, that it would be better to do Eclipse plug-ins for XUL.
However, this leads to the idea of the competition between XUL and Eclipse as platforms for writing cross-platform rich-applications, which interests me. Let's compare them as fair as my limited knowledge (not having written any apps in XUL or for Eclipse) can allow. Basically, in terms of flexibility and power of the widgets provided, Eclipse wins, hands-down. Especially useful to me, as a user, is the flexibility of the tabs and layout in Eclipse. However in terms of footprint, Firefox is the clear winner. The difference is simply enormous. An Eclipse-based web-browser, for isntance, would almost certainly be an unacceptable download size for your average user, and the memory usage would be killer too, most likely.
Going back to the tabs in Eclipse though: man, what I wouldn't give to have that ability in Firefox. I could essentially open the tabs and sidebars any way I like, and see as much stuff at once as desired. Sure would be handy some times. Oh well, just random musings...