Hard to describe so I've got screenshots. When you launch Firefox, there are several stages of ugly before the window is complete. The first problem is that the toolbar area and the content area are different colors. The toolbar area is white while the content area is gray. You can see this in the first and second screenshots, most clearly in the second. The second problem is that the toolbar white area extends up over the top of the top window frame. You can see this most clearly in the third screenshot though it is present in 1-3. There may be better fixes, but I think making the chrome and content areas the same color before and getting the chrome area not to break out of the top of the window frame would go a long way to making the launch experience not look so janky.

If you open the four images in tabs and then ctrl tab through them, it gives you a sort of slow motion launch animation. If there's any way to make the window look like its final state for those first three shots, that'd be ideal. If not, then at least unifying the color and getting that top window border matching the sides and bottom.

This is a dupe, I'm pretty sure, but the screenshots are good to have.