What you're seeing is called a content script, and it's how addons using the new webextensions API interact with web pages and provide a user interface. It's not a bug or some malicious code.

https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts

The Lite version is going to do the same thing but it's been depreciated and no longer receiving updates so I wouldn't recommend using it.