No Audio with Firefox Browser

Within the past week I took delivery of a new Dell XPS 8910 with Win 10 Pro to replace my older XPS 630i Win 7 machine.  I wanted more RAM than the older XPS could accommodate.  I purchased Zinstall to do the software and files transfer which took about 17 hours.  There were a few glitches, but in general I was pleased not to be spending many hours doing the transfers and setups one at a time by hand.  It's mostly working fine.  But I have no audio from videos posted on Facebook when browsing with Firefox.  Sound is OK with the "Edge" browser.  Other programs that generate audio work fine.  All of the windows sounds are OK and I have run every diagnostic I can find.  I've updated drivers.  I've disabled add-ons.  I've tried "Safe Mode".  No luck.  I've posted requests for help in Windows and Mozilla support forums. I have gone back to using my older machine until I find a solution.  Thanks in advance for any and all suggestions.
George Butch
Plantation, FL