Dark theme with a nice feel, but after a while, it starts looking sort of plain.

Fractal Plasma Jet Engine is an exceptionally clean, dark Firefox theme, with exceptionally clear text presentation in menus, tabs and toolbars and toolbars. I'm definitely sticking with this one long term. Lordsong has a numerous wonderful themes that you may also like to try. Stained Glass Fractal is another of my favorites.