How do I create an extension/add-on that will work for both Firefox and Chrome?
user
As of version 57, Firefox uses the WebExtensions API, which is largely compatible with that of Chrome. As such, you might be able to use your Chrome extension in Firefox without any major issues. Otherwise, you should use libraries such as Mozilla's WebExtension Polyfill to ensure that your code has the same functionality across browsers.
assistant
So i could just initially develop my extension for Chrome and then later just also use it in Firefox?
user
If your extension is simple enough, that might be the case. Even if it's not, though, you should only need a few modifications to fix it.
assistant