- Hackers specifically target IE users because of the many security holes IE has which allows infiltration to your computer. Firefox's security is top-notch.
- IE loads pages a lot slower (
Click here for the proof on that). Firefox isn't the fastest, but considering all its optional add-ons that IE doesn't have, it's still way faster.
- IE reads coding in a completely different manner. That's why a lot of sites are all, "Eff this shit; use Firefox if you want to see what my site *really* looks like" disclaimer. With all its unnecessary float bugs and div misreads, IE is ridiculous to code for.
- IE is far more likely to crash due to its instability. I'm sure there's a lot of people here that can attest IE randomly froze up and they lost their posts. Firefox saves anything in form format (such as typing up posts) where you can easily hit the back button, and your post is still there.
- Firefox allows modifications in terms of aesthetics (themes) and greater security--such as AdBlock (blocks all pop-ups + specified advertisements on websites), ScriptBlock (blocks Javascript which can be used maliciously), and 100's of others I haven't downloaded.
For the American comment, US-Cert (homeland security team) recommends Firefox to help U.S. security. Doesn't do that for IE.

Maybe once the latest IE rolls around (9? 10?) it'll be safer. I'm sure Google Chrome, Opera, and what not are just as good. Firefox just happens to be the "it" browser like IE used to be back in the day.