Recently, YouTube has started limiting resolution on Firefox on XP systems. If we try to watch a video, it is only available in 360p, and there is a "Missing Options?" choice. Clicking this button brings up a screen that says we can get resolutions higher than 360p by using Chrome or by using Firefox on version 7 or later of Windows.
Because high quality worked just fine on YouTube a few weeks ago, the change wouldn't seem to be a technology issue. I tried changing the User Agent string to indicate Windows 8, but that had no effect. So YouTube seems to be basing the limitation on some other way to determine the operating system.
I also tried using the Firefox Add-On "HD", and while some features of this add-on worked, it did not increase the resolution.
How can I get YouTube to play HD video on my XP box again?