uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do :

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.3 with a Full Version Installer

• https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

Still no sound. Firefox is installed on F: drive to save spavce on my C: drive.

Sarge1948 said

There is no sound on youtube or anywhere else only in firefox. Everything works fine in other browsers.

Are you using Windows 10 Insider builds?

If so the sound likely still works in anything using the Flash Player Plugin from Adobe in Firefox but not in HTML5 player.

Mozilla has a working workaround patch in upcoming Firefox 58.0 (Jan 13) and maybe for the Firefox 52 ESR as Microsoft may be a while fixing this on their end even with the cause known.

Bug 1426719 -Latest insider build of Windows 10 (17063) breaks sound playback completely Bug 1426285 - HTML5 no sound/audio

from 1426719 comment #38

Heads-up, Windows fix has reached mainline as of build 17073

Hi guys, this problem can be caused with the upgrade of Firefox if you are using a sandbox (Sandboxie). To overcome it a solution is as follows -

 Open Firefox outside Sandboxie
   Type about:config and accept the risk
   Search for "sandbox"
   Switch security.sandbox.content.level to 2 (mine was 3, if needed switch it to 1 or 0 ).
   Close Firefox
   Relaunch Firefox 57 Sandboxed and re-try the audio.