Mozilla's pop-up blocking is pure genius. It only blocks window.open commands INSIDE of an onload or unload event. This get's rid of 90% of the junk and keeps 90% of the good content, unlike Opera where my only choice is to get rid of all pop-ups, and it is surprising how many sites use legitimate pop-ups.

Mozilla's DHTML support, i.e. layers and Javascript, is still very flaky though. I work on a website now that is, to my regret, IE only, and that is because Mozilla can not handle the extensive use of layers that we have, as well as many other small issues that individually I could fix but collectively would be a major headache(just for instance, doing a window.location to a PDF file does not work right for some reason). I try every new version when it comes out and look forward to the day when I can say it is just as good or better.