Widevine is unrelated to YouTube. Widevine is only there to serve you protected content from Netflix, Hulu and the likes. YouTube works fine with the default HTML5 player. Unless you are using Flash (which itself has various issues), which from your System Details list, you are, then I would try disabling Flash and seeing how it runs.

As I said, when I turn off Widevine YouTube is fine, so obviously they are related somehow.

I tried turning off Flash and that had no effect on the problem.

You will need Widevine for Netflix as you probably know.

You can open the YouTube test page to check support in Firefox for playing HTML5 media.

• https://www.youtube.com/html5

See also:

• https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows
• https://support.mozilla.org/en-US/kb/fix-common-audio-and-video-issues

If you have any YouTube or media related extensions then try to disable those.

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• hold down the Shift key and left-click the Reload button
• press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• press "Command + Shift + R" (Mac)

Clear the cache and remove the cookies from websites that cause problems via the "3-bar" Firefox menu button (Options/Preferences).

"Clear the cache":

• Options/Preferences -> Advanced -> Network -> Cached Web Content: "Clear Now"

"Remove the cookies" from websites that cause problems.

• Options/Preferences -> Privacy -> "Use custom settings for history" -> Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Appearance
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Thank you but none of those things worked.

As I said, I did identify which add on was causing the problem, but I can't simply shut it off without causing another problem.