[dheretic]

Take a look at how many projects fall under the Mozilla platform then multiply them by 6 major platforms: Solaris, Linux, FreeBSD, Windows, MacOS Classic and MacOS Carbon. IE runs on only 1 platform well. Mozilla runs well, in my experience, on Linux, MacOS and Windows. I'm using a 1.0 nightly build right now to post this. Plus keep in mind the countless number of ports for OSs like BeOS, OS/2, etc.

[Frumious Bandersnatch]

I must reluctantly disagree with you on this one (sort of).  IE probably runs better on Macs than it does on Wintel.  Given that, however, I'd have to agree that porting Mozilla to various platforms is harder because it runs on so many of them.