I didn't notice a template for browser add-ons/extensions related to Emacs, just one instance Emacs as a browser's external editor. If a category for browser extensions is added later, please add this page to it.
Emacs-mode is a Chrome browser extension. It adds "Emacs bindings and quick navigation without using the mouse". Typing a "?" in a web page will launch a list of keybindings.
Emacs-mode does not mimic Emacs navigation perfectly: C-n still launches a new Chrome frame. It at least allows [C-x k] (or M-x kill-buffer) to close a browser tab, which is useful for those of us who have the command burned into our ROMs.