No one two installs of Firefox on different computer act the same. So unless the other Firefox install was bit/bit install with same Addons with different profile will not always do the same for the same Browser. And Edge doesn't tell much what is happening on Firefox but that is plays on Edge only.

You need to use Firefox in Private Mode windows Browser to see what if any Addon issues there are on the site or not. Then go back to standard with All Addon disabled and turn on/off each one to see which is the issue and if so stop using it or ask the Addon Developer for support why it cause Youtube issues.

What security software are you running? Have you tried under a different windows account on this computer? Also, download another version of Firefox (Beta, Dev, or Nightly), make sure to install in a different directory than your current install. https://www.mozilla.org/en-US/firefox/all/#product-desktop-release

Any relevant-looking messages in the Web Console ?

• https://firefox-source-docs.mozilla.org/devtools-user/web_console/

• 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
• https://support.mozilla.org/en-US/kb/html5-audio-and-video-firefox