YouTube videos black, only audio in Firefox

When I play a YouTube video on my computer, it seems to play fine, but the whole video is black. This happens only when I play the video in Firefox.

I'm using Windows 8.1

What I did so far:

• Restarting Firefox
• Reloading the page with Ctrl+F5
• Seeking back and forth in a video
• Playing different videos, with and without playlist
• Cleaning the cache, cookies, history and everything else...
• Updating Firefox to the latest version
• Disabling all extensions
• Deactivating all plugins
• Opening the video in a private window

Still the same...

I don't want to restart my PC or uninstall and reinstall Firefox, there must be a way around that.

And before I forget:

• The video ads are black too.
• HTML5 videos on other websites work fine.
• I can see all the thumbnails of the video when moving the mouse over the seek bar.
• The videos are not opening in Flash player through the browser - they are the HTML5 versions

Solution 1:

The Mozilla support forum has an entry for this problem and it relates to the browser using hardware acceleration causing the problem.

Source: https://support.mozilla.org/en-US/questions/1044374

1. Click the Firefox menu button
2. Click Options
3. Click Advanced
4. Under General untick Use hardware acceleration when available

If you disable hardware acceleration it will fix the problem but it may cause problems elsewhere with, for example, WebGL and other browser graphics technologies.

Solution 2:

This question describes disabling all extensions and plugins and still having the problem. In that case, Big Chris's answer is a pretty good bet for the solution. For people who have not tried disabling extensions, and that answer doesn't fix the problem, there is a good chance that extensions are an additional issue.

In my own case, many videos (at many web sites), display just a black window (not even audio). Some are HTML5, some are Flash, and hardware acceleration is off. The issue turned out to be that many sites won't work if ads and/or trackers are blocked. For example, I use Adblock Plus to block pop-up ads and Ghostery to block trackers.

So far, all problem sites for which Big Chris's solution didn't work, were fixed by whitelisting the site or pausing blocking on one or both of those extensions.