drhowarddrfine - 7:13 pm on Mar 23, 2010 (gmt 0)
I've found it better to check in Opera first. While Firefox does an excellent job of touching all the bases, they sometimes don't have 100% coverage or they let errors slide through you wouldn't otherwise catch. Opera doesn't cover all the bases but, what they do cover, is usually very thorough.
I trust that by developing to Firefox/Chrome/etc. that Opera should behave correctly for the most part
For example, I've been working on a web app that's served as XML. The first few pages looked great in FF but, when I checked later in Opera and Chrome, I got the yellow XML error screen. My XML was not well formed at a couple points. Other times things were slightly askew with my CSS that needed minor adjustments. (Note: these were minor things and not any major problems like in IE which failed hard.)
As for IE6 support, I now charge a 30% premium for anyone wanting their pages to work in it.