(Please update to Firefox 62, 60 is out of date and not supported).

You can customize the way Firefox looks if you like! I personally, love the dark mode :)

Customize Firefox controls, buttons and toolbars Use themes to change the look of Firefox

Hi RozettaL, I don't know all the history behind squaring the tabs, but the old design was composed of lots of little image files to create the edges and icons, and the new design does away with most of that and uses resizable vector-based images which improve the drawing speed.

As Tyler notes, people have come up with ingenious methods for reshaping the tabs and plugging in icons they like better. But it's probably a few hours effort to do a complete makeover, so you might tackle a couple changes at a time based on what's most important to you. Let us know where you want to start, or whether you want to give it a little time to see whether it grows on you.