Hi, please provide more detail such as the URL to the site if no private information is needed and what add-on as no information came with your question. Also please be expand on the issue more. Thank You.

This occurs on every page with pdf file. This is my created addon which reacts to window onload event and adding url to title. It worked properly to update of 60.0.1, and when I'm entering to page with pdf file it's not working now. I temporary created also a test addon which creates alerts every two seconds, and it working correctly until i enter to page with pdf file like f.ex : http://www.pdf995.com/samples/pdf.pdf

This bug can be connected with the issue.