Operetta Advance theme makes Firefox look like Opera 10
Up until today I've been using a dark Firefox theme. It looked quite pretty, but eventually I got tired of not being able to read the black text on some of the dialogs (yes, I've tried tweaking userchrome.css -- multiple times).
So I've set out in search of a lighter theme which would still be nice and minimalistic. Operetta Advance seems to fit the bill perfectly. It's called "Advance" because the same developer also put out a version called just "Operetta," but I couldn't find what makes them different. Naturally, I went for Advance. That's marketing for you.
It's quite elegant, and manages to make even my cluttered Firefox seem a bit more unified. My only gripe with it so far is that the gripper right under the tab bar does nothing. Firefox doesn't have those cool expanding thumbnail-tabs Opera has, so I think it just shouldn't be there.
Operetta Advance is currently experimental, but I warmly recommend it.
Feel free to post your favorite themes below, especially if they're light and minimalistic.