You're describing userscripts, which is what site focused web extensions essentially are anyway. Install the Grease/Tamper/Violet monkey extension and you can f

You're describing userscripts, which is what site focused web extensions essentially are anyway. Install the Grease/Tamper/Violet monkey extension and you can find scripts from Greasyfork.org, or author your own. Each script defines a scope listing the sites it's allowed to run on.