I accidentally inverted the color of my Firefox tabs

[ratcreature]

Does anyone know how to undo this? I must have pressed the wrong key combination, but I don't know what I pressed (I wanted to press Ctrl+T to open an new tab), and now instead of dark on light my Firefox tabs except the active one have light writing on dark background. It's a bit disorienting. I can't find a corresponding setting in the menu.