As I mentioned before, we are back to investigating some gfx deceleration issues. Preliminary investigation shows that our border+gradient code is inefficient and since Firefox uses these features heavily, we get some epic slowdowns when tab switching.
To test out this theory Bas put up a test build that simply does not draw gradients or borders. See bug 75087 for a screenshot demonstrating drastic reduction in browser attractiveness. There is also a test build for people suffering from slow drawing to try out.