the listener function in background.js is not doing anything and not used anywhere. To keep things simple, I suggest removing it now and adding when it is needed.
The extension causes several websites to crash such as youtube. This is caused after injecting the new html script after detecting the DOI's. I noticed that this crash is caused due to deletion of some HTML elements. What I suggest is carefully trying to study how the code changes the script and try avoiding the deletion of elements.
After the last push to main, index.html would not appear to the user when the extension icon is clicked. This has to do with the way panels are organized in the popup. Since it is not easy to add new panel, I would suggest adding the copyrights section under the 4 current boxes(Findable, Interoperable .etc)