Puntuado con 5 de 5 estrellas
I like this one best because it results in smaller fonts in the Add Ons Manager too.
I suspect it combines with some other add ons such as Theme and Font Sizer Changer to do this but whatever the reason some other alternatives don't manage this.
My only request really would be for a "Toggle Slim/Normal" item in the Add Ons Manager settings menu but that's possibly being a bit picky : )
You can just disable and re-enable the add-on quickly as it is restartless! Would that work for you?