You can check the media.*enabled prefs on the about:config page and possibly reset user set (bold) via the right-click context menu to the default value.

• media.webm.enabled
• media.mediasource.*

Note that it is best to be cautious with toggling prefs to enable features that are off by default. They may be off for a reason like no proper support on your computer.

That video is available in MP4 and WebM format if I check the code in the Inspector. I get the WebM version on Linux via copy video location.

• http://fat.gfycat.com/FineJubilantBoubou.webm

Still green in webm. Webm's and HTML5 videos were working just yesterday.

So the WebM link I posted above isn't working?

Nope. I can hit "play," but it's still a green screen.

Try to delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in current Firefox releases.

• http://kb.mozillazine.org/Preferences_not_saved
• http://kb.mozillazine.org/Resetting_preferences

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Started up Firefox in Safe Mode. Looking at YouTube's HTML5 page, H.264 and WebM VP8 were no longer red, so that means that the problem is an extension, correct? However, when I tried the webm above, it was still green.

EDIT: Oh wow, I'm an idiot. I had the "YouTube Flash Video Player" extension enabled. So now on the YouTube page, everything's blue.

HOWEVER, webm's and HTML5 content still gives me a green screen with a ringing noise.

EDIT2: Okay, found the solution. Had to set media.fragmented-mp4.use-blank-decoder to "false" in about:config and now everything's blue on YouTube for the first time and HTML5 content is working again.

Thanks for the assistance.