Learning Synths is free and works on any browser. You can start with the basics, or if you already have an understanding of amplitude and pitch, you can jump right into sound design. The lessons are presented in a very Ableton-way, with monochrome geometric shapes that users can manipulate to produce different sounds. Don't be fooled by the simple interface, though. While the lessons are straightforward, they contain plenty of substance, and you can play around with envelopes, oscillators, filters and more.
A couple years ago, Ableton introduced a similar tool, Learning Music, meant to teach users the basics of music production. That also included mini-lessons, on things like beats and notes, and it featured deconstructed real-world songs like "All the Single Ladies" to experiment with. Like Learning Music, Learning Synths is simple and easy to follow, and there's a good chance you'll walk away with fresh (or at least refreshed) synth insights. Maybe you'll even put them to the test on other Ableton products.