The Toolbar

Pop-up blocking - Numerous programs are available for IE that can do this quickly and easily, as well as provide a multitude of other features. The Google Toolbar is probably the best.

Search on Navigation toolbar - Google Toolbar does this too...

Tabbed browsing - Uh, yeah, woo. So the open pages appear below the browser's toolbar instead of on the taskbar. Why is this considered such a great feature? IE does the same. Additionally, if you've got lots of pages open, Windows XP will collapse them into only one taskbar button for you, from which you can select which window you want to open. MDI is dead -- get over it.

Download window - I don't want a single download window, that sucks. Status windows for each window are much better. If you seriously have trouble remembering where you saved things then you probably shouldn't be attempting to install a new browser.

Themes - IE supports theming -- in fact, you can take its whole engine and use it in a browser of your very own (see: Crazy Browser, MyIE2). IE also supports toolbar backgrounds and you could probably skin it easily enough using something like WindowBlinds if that's really your thing.

Extensions - I'm sure you'll find many, many more extensions available for IE than for Firebird, both freely and commercially. As the world's most popular browser, it makes sense that people will develop any web browsing extension available for it. IE is the standard.

Firebird often crashes, and it's slow. It doesn't work with every page you'll come across, unlike IE which obviously will (because they were designed for it). Firebird's UI is clunky and non-standard (on Windows at least), as it doesn't use the normal Windows toolset. IE, by contrast, is snappy, simple and standardised.