Not sure if this answers your specific question, but you can limit the sites that an extension can run on. I recently discovered that Chrome offers this feature (right click the extension icon and select Manage Extension to access), and it saved me from having to build a site whitelist feature for my extension. [1] It already has a blacklist feature, and I was going to build a whitelist feature due to user requests. Then I discovered that this functionality is built into all Chrome extensions.

Unfortunately it doesn’t seem to exist on Firefox.

1: https://chrome.google.com/webstore/detail/beeline-reader/ifj...