Thanks for your rating! When displaying PDFs with the built-in PDF viewer the usage of the extension is not possible, cause firefox blocks that due to security reasons.

Simplest workaround: Open the PDF on the page "https://mozilla. github. io/ pdf.js/ web/ viewer.html". Remove the spaces in the link - links are not allowed in comments. "Open" in the top right menu. Cause that is an external page, the addon works on it.