Using Tor Browser 7.0a3, I visited a page that had a video ad (happened to be [this one](https://arstechnica.com/gaming/2017/04/after-nintendo-switch-does-the-3ds-have-a-future/)) and got a banner about PulseAudio being required:  I clicked the "Learn how" button, and it led me to this page, which is 404 "Page Not Found". https://support.mozilla.org/1/firefox/52.1.0/Linux/en-US/fix-common-audio-and-video-issues ([archived copy](https://archive.fo/BCXOW)) After you have clicked the button once, you can make it appear again by setting the pref `media.decoder-doctor.MediaCannotInitializePulseAudio.button-clicked` to false.