"Apparently, it's because Linux users want it that way:
The reason Linux isn’t shown above is that all of the feedback we’ve received so far indicates that Linux users would be happier with a theme that uses native GTK icons in the navigation toolbar, which rules out this type of customized visual treatment."
"I was never a huge fan of the icons on Firefox Linux, so I installed the Camifox skin, which is really nice. I'd also suggest reducing the system font size (System > Preferences > Appearance > Fonts). The default on Ubuntu is very large compared to Windows. In that dialog you can also play about with the theme, which Firefox will inherit.
There are loads of other browsers you can install on Linux. Opera is a solid choice, though its default skin is worse than Firefox. Google Chrome v3 is also available - technically in alpha stage but I haven't had any problems with it.
If you really want to you can install IE6 on Linux! Look up ies4linux."