Moving to Firebird/Phoenix
I moved to Firebird about a month ago. I've had some sort of gecko based browser on my machine for a while, mainly for testing out web pages and making sure they worked in non-IE browsers. After a while I found that if I designed solely for Gecko browsers, then the sites would look fine in Opera/Gecko and usually IE. The only problems I had where simply bugs in the IE CSS rendering engine, and it looks like implementing something like max-width can't be done without a change to the underlying OS.
The other day I went and set Firebird to be my default browser. I deleted the IE link in quicklaunch bar. The only IE I use is when an app embeds the IE control in the app. See the this post from Charles Cook on embeding Gecko in .NET apps.
The day IE 6/7/8 actually renders css/edge I may switch back. Until then, Firebird is just a better browser.
Edit: for comfirmation, the max-height stab was done in sarcasm. I guess IE is having trouble rendering the <sarcasm> tags :-). My point is that fixing CSS bugs is not something done in a service pack, you need IE 6.5 if you are changing the way it renders elements. IE 6.5 doesn't appear to be coming our way.