Before I tell you why I think that is the case, I feel that I need to clarify that I'm actually a Firefox user and have been since before it went final. Not an avid Firefox user (there are things I don't like about it), but reasonably happy.
Internet Explorer's late introduction of proper translucent PNG support was (and to some degree still is) somewhat of an annoyance. However, Internet Explorer had support for "flat" alpha blending through CSS filters for a long time. Actually long before similar features showed up in other browsers. Nowadays, most browsers support the "opacity" property which is likely to become an official standard with CSS3. Most browsers, except Internet Explorer. I could point out the irony in this, but do I really have to?
Look everybody, Safari supports the CSS3 property "text-shadow". How awesome is that! Well, I always thought drop shadows were kinda lame, but then again I'm not a designer. Of course Internet Explorer has supported drop shadows and all kinds of text effects for quite some time. Admittedly, the drop shadows in Internet Explorer are not nearly as sexy as the ones in Safari and they are built on proprietary CSS extensions, but it's the thought that counts, right?
For the longest time, Internet Explorer was the only browser to support iframes. In fact, iframes started as a proprietary extension to HTML which later was adopted by the W3C and incorporated into the HTML4 standard. The use of iframes has declined over that past couple of years, but they're still invaluable in web applications for asynchronous file uploads, HTTP streaming, widget isolation, etc.
Asynchronous HTTP requests
Most designers as they go through design school are taught about the significance of fonts, about different font styles, about serif and sans-serif fonts and things of that nature. Okay, I'm talking out of my ass here: I have no idea what designers are taught in design school, but I'm assuming that fonts play their part. Either way, I can say from my personal experience that when you tell first-time web designers that they can only use certain "web-safe" fonts in their designs, they're not particularly happy. Unwilling to accept these limitations, designers often feel compelled to put text into images or, even worse, Flash movies. Now here's a crazy idea: wouldn't it be nice if we could somehow embed the fonts used by a particular website into the website itself? As it turns out, recent versions of Firefox, Safari and Opera support this feature through the CSS3 rule "@font-face". Internet Explorer on the other hand has supported this since 1997. The catch? Internet Explorer only supports embedding of special rights managed font files while Firefox and the likes only support embedding of un-managed fonts. Why the rights-management? So that if you embed commercial fonts, Adrian Frutiger doesn't sue yo ass.
Microsoft's decision to suspend all Internet Explorer development after the release of Internet Explorer 6 has turned out to be a fatal mistake. Internet Explorer 6, while it may seem old and dated today, was actually on the cutting edge when it was released. Eight years later, Internet Explorer has lost significant market share and has gone from innovating to playing catch-up. That's a real bummer because Microsoft had all the right ideas about dynamic HTML and web applications. In our wonderful capitalist world, innovation is often driven by competition. When Internet Explorer 6 was released, it quickly gained a market share of over 90%. The end of competition meant the end of innovation. Now that the competition is back, so is innovation. Microsoft isn't on the forefront of it, but who knows where this is going. As long as I'm on the receiving end of this new, much more civilized browser war, I'm all for it.