Gotta get better DOM support in them, first.
AFAIK, there's one page that don't go in IE 5.1. Once that's fixed, the next in line is either NS6.2 or Moz 1.0. Accomodating that may be harder, as there are some layout issues - basically, we've coded to IE's layout bugs and it's about to come back and bite us. :-/
Opera is a Later Thing as it doesn't (yet) support two or three key pieces of DHTML yet. I believe I've mentioned that elsewhere here on IWETHEY and yes, I am disappointed. Fortunately, Opera is aware of the lack, because lots of people have requested these features, so support for them is coming!
Wade.
"All around me are nothing but fakes
Come with me on the biggest fake of all!"