Have you turned off D2D? & Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:

• Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

• Upgrade your graphics drivers to use hardware acceleration and WebGL

There is a bug for this that stopped tracking in version 25, let me find it:

• Bug 891916 - Vimeo videos image is delayed
• and/or [Bug 984396 - High power consumption on Vimeo ]

I am pretty sure these could use more info on where and when this happens. Please feel free to vote on them too.

Like I said, I am able to view videos in safe mode. That means that my display drivers are working fine.

I have followed the directions in the links and disabled all extensions manually and I am already running the default theme. Yet, the problem persists. However, it does not persist in safe mode only for some reason.

I have used the following link to test the playback : http://www.quirksmode.org/html5/tests/video.html

Here, the WebM and the Ogg/Theora video plays fine but the H.264/MP4 video does not. I can hear the audio, but there is no video.

I will post a screencast later.

Turning off hardware acceleration may improve this issue, however please also check to make sure the webgl is ups to date: it looks like it is. Upgrade your graphics drivers to use hardware acceleration and WebGL

These troubleshooting features, are they still the same?

direct2DEnabled: True

directWriteEnabled: True webglRenderer: Google Inc. -- ANGLE (ATI Radeon HD 4800 Series Direct3D9Ex vs_3_0 ps_3_0)

issues for this driver: https://bugzilla.mozilla.org/show_bug.cgi?id=911279 CURRENT WORK AROUND for text rendering https://bugzilla.mozilla.org/show_bug.cgi?id=812695#c414

As I have mentioned before, hardware acceleration is already turned off.

I have found these :

gfx.direct2d.disabled : false gfx.font.directwrite.enabled : false

I could not find the remaining option in about:config

I do have a Ati Radeon HD4850 video card. However, this does not explain why the video plays without any problems when the browser is running in safe mode.

Ah, is it Flash player only? Your plugin is up to version 12 (Shockwave Flash 12.0 r0 ) There was a security update to 13 that may have also improved video quality. http://www.mozilla.org/en-US/plugincheck/

For issues with HTML5 videos, you can try to disable the HTML5 media player by setting a few prefs to false on the about:config page.

• media.windows-media-foundation.enabled = false
• media.directshow.enabled = false

You can open the about:config page via the location bar and you can accept the warning and click "I'll be careful" to continue

• http://kb.mozillazine.org/about:config

I have updated Flashplayer to version 13.0.0.206 (latest).

When I set "media.windows-media-foundation.enabled" to false, the videos started playing. But some videos (such as the ones on Vimeo) show poor performance with low framerates. Setting or unsetting media.directshow.enabled does not matter. What could be the cause of the poor performance?

When you disable the HTML5 media player then the website will fallback to another media player, likely Flash based, so you can suffer from Flash issues instead.

You can check for problems with current Shockwave Flash plugin versions and try this:

• check for updates for your graphics drive drivers
  https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration
• disable protected mode in the Flash plugin (Flash 11.3+ on Windows Vista and later)

*disable hardware acceleration in the Flash plugin

See also:

• http://kb.mozillazine.org/Flash#Troubleshooting

I have the same problem, except on a Mac running Firefox 31. I even ended up testing MP4 video playback using the exact same website the original poster did, before finding this thread. All the same symptoms, and restarting without add-ons fixes it. I've tried configuring add-ons like Flashblock and NoScript NOT to block HTML 5 and MP4 video, but that still hasn't solved the problem. Only restarting without add-ons solves it. I've tried everything people have suggested here, and those suggestions haven't helped, either.