Most modern web browsers are interoperable. i.e., what you see in IE7 (what I use) is what you should see in FireFox. For the most part.
As a webmaster, I've no idea what you're talking about.
I agree. "Most" webmasters don't deliberately include FireFox-specific code that will "break" Opera or Konqueror or whatever. In fact, I'm not too sure that there
is that much FireFox-specific code. Maybe some strange JavaScript functions, or prematurely deprecated tags.
Browser-specific code is sort of anathema to what the FireFox/Mozilla project is about.
Now, on the other hand making pages specifically for IE... THAT is a crime. Especially when one uses annoying, deprecated tags as:
<MARQUEE SCROLLAMOUNT=100 SCROLLDELAY=50>LOL EYE AM KEWL EYE CAN MAIK A MARKEY!!1!!</MARQUEE>