Is there a way to go back to the previous browser look?

Today my Firefox browser upgraded to an "improved" look, but I hate it. The little square containers that used to be unobtrusive are now huge and visually distracting balloons, meanwhile it's harder to tell where the tab borders are or what tab I'm on. And the faded color in the top bar make it hard to read the text to the left even though I picked the darkest option, but I don't want to install themes by random makers. I just want to go back to how it was, it was fine that way, you don't have to constantly change things just for the sake of changing them.