If you want to build a “content script”-type extension that does nothing but interact with the page and run normal JS, think about building the extension on the GreaseMonkey platform. GreaseMonkey (http://www.greasespot.net/) is an extension that lets you write a content script as a single JavaScript file, with metadata such as the name and version written in specially-formatted comments at the top of the file. That makes scripts easy to develop and install. GreaseMonkey also provides a few functions that allow you to do some things that browser security doesn’t normally let you do – http://wiki.greasespot.net/Greasemonkey_Manual:API.

“GreaseMonkey” is only a Firefox add-on. But there are GreaseMonkey-compatible extensions for other browsers, such as Tampermonkey for Chrome (https://chrome.google.com/webstore/detail/tampermonkey/dhdgf...) and GreaseKit for Safari (https://www.macupdate.com/app/mac/20718/greasekit). Thus, GreaseMonkey scripts are automatically cross-browser. Chrome even supports installing GreaseMonkey scripts natively, but only through a convoluted process, for security reasons. The reason people made clones for other browsers is the huge library of existing GreaseMonkey extensions at https://userscripts.org/.

The downside of GreaseMonkey is that the user needs to have a GreaseMonkey-compatible extension installed to run your script. So your extension depends on another extension. It’s also harder to use a build system while writing a GreaseMonkey script, if you want to write it in CoffeeScript or something.