SICP. I'v learnt about lazy evaluation/streams wonderland, and got impressed by its abstraction power. This technique lets you express some mathematical abstractions on series elegantly and concisely. It's the same elegance that you get in physics when representing three-dimensional time evolving systems as a set of world-lines in four dimensions: mind-boggling at first, but enlightening once you grasp it.
The latest releases of Mozilla seem to be getting usable, and i'm giving it a(nother) try these days. It is a resource hog, though, and i must fight hard against my aversion to bloated software: i still use links for browsing local documentation.