Where is that link coming from? Links to YouTube usually start with HTTPS, not RTSP. If you are using a video recording or video downloading add-on, perhaps that is doing something with the links?

I have a big collection of bookmarked music - Jimi Hendrix, JS Bach, etc. They all worked yesterday, but now today I get this message. THE LINKS all work wiehn copied and pasted into Chrome or IE, but they don't work in Mozilla Firefox. Here is one of the links: https://m.youtube.com/watch?v=h94gvPDapvw

Are you using any extensions that deal with YouTube or media files?

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You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.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

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

"Remove the cookies" from websites that cause problems.

• Firefox/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