Why Firefox on Linux looks different than Windows/Mac?

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.

Firefox doesn't suck on Linux. It might eat up some resources, indirectly, if you don't use Gnome or GTK and have limited hardware. Or perhaps you've lucked out with a bad build/hardware combination. But from my limited experiences, I don't see a problem.

Edit: Looking at your updated question, I'll say this much: that's different HTML. I get the same image (balloons) every time, so it's not just random. For some reason they chose to have a different background image and layout for each platform; maybe the fonts are slightly different but that's a non-issue.

Or if you're talking about the GUI itself, well, I'm glad it changes per platform. UI integration is always a good thing.

It's a single core code base but there are different pieces of code for platform specific features (UI, for instance). No it's not completely Javascript. There's plenty of C++ in it.

Each version uses a different front end. For example on Mac OS X firefox uses a Cocoa UI - which means it can integrate with other Cocoa applications and the system more easily than if it were written with another API.

Depending on what GUI APIs are used on Windows on Linux (I don't know about windows, but I assume Linux is GTK+) you will see different levels and experiences of integration.