So, it looks like there's a number of things you're confused about. First, Atom doesn't have anything to do with Mozilla's Ace or Bespin. Ace and Bespin (now Cloud 9) are tools that live on a server, which you access through a browser.
Atom is a completely separate project (started by Github, not Mozilla) to build a native text editor using web technologies. The text editor is a native application, but it exposes a Javascript API for plugins. After a while, Github realized that the infrastructure needed to make Atom work (e.g. installers, updates, notifications, etc.) could also be used by other projects, so that code was split off into "Atom Shell". It's this project that's getting renamed to "Electron".
Atom is a completely separate project (started by Github, not Mozilla) to build a native text editor using web technologies. The text editor is a native application, but it exposes a Javascript API for plugins. After a while, Github realized that the infrastructure needed to make Atom work (e.g. installers, updates, notifications, etc.) could also be used by other projects, so that code was split off into "Atom Shell". It's this project that's getting renamed to "Electron".