Also--hackers are continually finding new (or previously undiscovered) flaws in Microsoft's programs
And now that much of its code has shown up on the 'Net, they're only going to be able to exploit Windows' vulnerabilities even more...
There are a few things about Mozilla--mostly the way it displays certain graphics--that aren't quite as pretty as IE, but the trade-off on performance and the fact that it isn't vulnerable to the IE/OE exploits has made it more than worthwhile.
The converse is true as well. IE (even IE6) can sometimes completely screw up certain Web page presentation code. It's enough to drive any Webmaster insane. :- And if Microsoft sticks to the bizarre notion of never updating IE again, things are only going to get worse as new presentation features are developed.
As far as Mozilla goes, it works really well and has been garnering some of the best industry reviews...