This Motsenbocker's Lift Off 4.5 oz. Silicone, Latex Caulk and Foam Sealant Remover safely and effectively removes fresh or cured sealant from surfaces. The remover is water based and Scientific Certification Systems Certified - Biodegradable to protect both the environment and the user.
Nothing else was moving the old silicone around the shower door we're replacing, but this made easy work of it. For the vertical sections, I just rubbed it on with a paper towel and left it for a few minutes and that was enough to get it unstuck.