Aren't they amazing! (both squirrels and the feeder) I've had one slide down the glass section like that, too. But he fell off after just a few seconds and I never saw him do it again.
You're right, I was getting a lot of black snakes around when there was all that seed on the ground, too. But I hung a metal hoop with some window screening in it, so that it catches most of the spilled seed. Now the squirrels have all that seed to feed on so they really are not that motivated to steal from the feeder, and there is less feeding on the ground so fewer snakes, too.