Firefox 57 supports "lightweight themes" which are background images for the toolbar area. In the Customize panel, there is a Themes button at the bottom which shows the built-in themes, any you've added, and a link to find more. See: [[Customize Firefox controls, buttons and toolbars]]. Changes to the shape of tabs are, as you would expect, trickier. What kind of shape are you looking for? You can use custom style rules to round the top left and top right corners, or you can find sets of rules that make more comprehensive changes. A good site for presenting your wish list and seeing what people come up with is: https://www.reddit.com/r/FirefoxCSS/