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Firefox recently updated to V57.0.4 on a Windows 7 machine. YouTube videos now play a double soundtrack.

  • One soundtrack starts automatically as soon as the video loads, prior to clicking the play button or completion of visual rendering.
  • When the play button is clicked, or in some cases when visual rendering is complete but without clicking Play, both the video and audio start.

So there is the normal video/audio plus the duplicate audio track that started on its own some seconds before.

The videos play normally on other browsers on that machine, and on Firefox V56 on other machines. So far, YouTube videos are the only ones found to have this problem.

Note: this is a self-answered question, so I won't include all of the diagnostic details in the question, itself. This is intended to share the solution with others encountering a similar problem.

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The issue was competing add-ons. Details:

  • Several extensions were added some time ago when Flash Player was becoming a problem and the transition to HTML5 was underway. Different extensions had different effectiveness for different videos, and I loaded several for evaluation, hence the multiple add-ons. The extensions coexisted peacefully and several were left enabled with no problem.

  • The add-ons were all compatible with Firefox V57, so none were disabled as "legacy" extensions with the update to V57.

  • The update to V57.0.4 appears to be the change that triggered the symptoms.

    The problem was due to two add-ons:

    • YouTube Flash Player by Maxime RF forces videos to use the Flash Player

    • HTML5 Video Everywhere by lejenome forces use of the Firefox native video player.

  • When both add-ons are enabled under V57.0.4, they both work simultaneously, playing just the soundtrack in one format, and the video and soundtrack in the other format.

  • Neither add-on is now needed, so I removed both. However, if other users feel the need for an add-on, I actually had other related add-ons loaded as well. The only combination that caused the problem was the two listed above enabled simultaneously under Firefox V57.0.4.

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