After a colleague convinced me to downlod a copy of Mozilla Firefox, claiming that it was the "Internet Explorer killer", I have to say, I was sorely disappointed, for a couple of reasons.
1. Broken skin/theme support.
One of FireFox's trumpeted "new features" is skin/theme support. As you can see from the screenshot, it's broken. I tried three skins; unsuccessfully. It turns out that it's fine if you restart the browser, but that's beside the point. The interface doesn't tell you that it needs restarting.
2. While not browser realated, as a web developer, you hear both sides of the coin. Mozilla/etc browser users always complain that your site is "Designed for Internet Explorer", and doesn't display properly in their browser. They claim that because your site isn't standards compliant, it shouldn't exist.
I show you; a page from FireFox, displayed in Internet Explorer:
In a similar way to FF's skin support, it's broken.
"Maybe it's just Internet Explorer", i thought:
Nope, it would seem that the FireFox page authors themselves shun the standards. If I ever hear another Mozilla/Gecko/Firebird/Firefox user complain about standards non-compliance, I'll wring their neck.
Some other things..
3. While they've dramatically improved FireFox's load time, it's just not on-par with IE. I know; several components of IE are loaded with the Operating System, but you're not going to get the masses to use a new browser if they have to wait.
4. The interface is gluggy. In a similar manner to almost ALL Linux (or Open Source in general) interfaces, FireFox has a gluggy interface. It took me ten minutes to figure out how to move the Address bar to the line below the back/foreward buttons. And how do I turn off the damn Google toolbar?? (For those who are wondering: Create a new toolbar, drag the Google one to it, then turn the toolbar off).
Internet Explorer users are used to the ieda of being able to drag their toolbars, rearrange them, etc; without having to Right click and hit Customise. Don't want the Google toolbar? You don't have to create a new toolbar for it and disable it -- it's counter intuitive.
While many people think IE and Microsoft are the spawn of the Devil; I still believe that IE is a great, easy to use product, and until someone comes up with something better (And yes, I've tried Opera, Safari, etc), you're not going to knock it out of First Place.