I’m on a Mac & wasn’t getting the visual editor in 2.5.1, a fresh install, in Firefox or Safari. After trying half a dozen suggestions from the forum with no luck, someone mentioned something about a windows unzipping problem. I’m actually on a Mac, but this fixed me up:
I deleted everything, then downloaded the .tar.gz version instead of the .zip version (which is what I’d originally used), installed the usual way, and voila, visual editor is back.
