It's had one for years, and it's enabled by default. Firefox will automatically download a binary diff in the background, and apply it the next time you start the browser. If you don't restart for a while, it will eventually prompt you to restart. (Much like Chrome, except that Firefox currently uses a dialog box prompt while Chrome has a more subtle toolbar icon.)

Previously there were two types of releases: "minor" releases which were installed automatically because they contained security fixes, and "major" releases which prompted the user to choose first.

Now there will be no more "major" or "minor" releases. All new releases will be installed automatically, though I believe there is an exception if any extensions are broken by a new release. (EDIT: I mean, if any extensions that you already have installed will be disabled by the update.)