Sorry the numbering is supposed to be (1) - (4).

Update your plugins

Check that you have the latest versions of all your plugins.

   Go to our Plugin Check page and follow the links to update any plugins that are out of date. 

Allow Plugin Container in your Internet security program

Allow your Internet security software (including firewalls, antivirus programs, anti-spyware programs, and more) to trust plugin-container.exe also known as Plugin Container for Firefox. For detailed instructions, see Configure firewalls so that Firefox can access the Internet where in that case Firefox means Plugin Container for Firefox. Disable a conflicting plugin

If you have more than one plugin installed that can handle the same media type, this can cause a conflict that prevents some embedded media from playing. You may be able to fix the issue by disabling one of the plugins.

   At the top of the Firefox window, click on the Firefox button, and then click Add-ons. The Add-ons Manager tab will open.
   In the Add-ons Manager tab, select the Plugins panel.
   In the list of plugins, select the plugin you wish to troubleshoot.
       If you want to disable the plugin, select Never Activate in its drop-down menu.
       If you want to re-enable the plugin, select Always Activate in its drop-down menu. 

For example, you may need to disable the VLC plugin to play embedded QuickTime or Windows Media content. For more information about troubleshooting problems caused by plugins, see the Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems article. Other solutions Clear the cache

See How to clear the Firefox cache. Disable interfering extensions or ad-blocking software

   Ad-blocking software, such as anti-banner filters of your Internet Security program, or extensions, such as Flashblock or Adblock Plus, can prevent audio or video content from playing. If the site works with ad-blocking disabled, you may want to add the site to your whitelist in your adblocker filter.
   Any Firefox extension can also block plugins. If the video or audio plays when you disable all extensions, then one of your extensions was causing the problem. For more information, see Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems. 

Check some Firefox settings

   Verify that cookies are enabled in the Privacy panel in the Options window. Verify that the site is not blocked in the Cookie Exceptions list.
   Allow the website as an exception in the Firefox pop-up blocker. The "Block pop-up windows" option is found in the Content panel of the Options window

)

Ensure you enable hardware-accelerated in flash player. If that setting is shaded out make sure you have the latest video card driver installed on your system.

Hi, as I mentioned I disable all my addons to no avail, i.e. the problem still persists.

I turned off my Antivirus and restarted firefox, but the problem is still there.

Also I have tried turning hardware acceleration on and off in firefox, but this makes no difference.

Edit:

I have noticed that when I try to change any Youtube video to Hi-def it goes black and the video won't play, but if I then play another video from the side panel (on the right), it then starts in HD. Not sure what this means though. But I didn't have this in FF 23.0.

Thanks.

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Firefox/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

Look:

http://googlesystem.blogspot.com/2013/10/youtube-shows-if-your-browser-supports.html?utm_source=dlvr.it&utm_medium=gplus

Thanks for the help guys.

Clearing the cache didn't work, but removing the Youtube cookie and restarting firefox did. I can now switch to Hi-def again

Cheers :)

What I did was (not in specific order):

- clear cache (cache being at Override - 1024MB) - Hardware acceleration off - Turned off all Add-ons that I didn't need (ie: all except for Flash and Shockwave). I think Nvidia's 3D vision might've had a role in it. - Cleared cookies.

Now, 720p works for me.