Are there any tools for building cross-browser extensions? What would be really useful is a tool that can help you build a Chrome extension, Firefox extension, and Safari Extension.
while the actual work of the extension may be done in a web app, extensions get more access to the interface of a browser than a normal web app
example: the Buffer extension could probably just be a button that simply loads a JS file from their servers that does all the real work. WIthout being an extension, you wouldn't have the convenience of having the button on every page you view.