Firefox Addon Javascript doesnt get executed when opening a PDF file in the browser

Question

I am trying to write a Firefox Addon which enters the Password to a protected PDF on a website automatically. The Javascript to do this is very straightforward & already works perfectly if entered in the Browser command line. It just doesnt get executed when relying on the Addon.

I tested by replacing all the Javascript with just console.log('Works!'); If i enter any part of the site normally, the String indeed shows up in the console as expected. But if i open the link to the PDF(the PDF is stored on the same website), it simply refuses to execute. I dont even gey any error messages or anything.

Any hint in the right direction would be appreciated!