If not there yet please update to 61.0.2 by going to Help --> About

See if anything here is helpful : VIDEO AUDIO

• https://support.google.com/youtube/topic/3014746 - Youtube
• https://www.youtube.com/html5/ - Can you support HTML5
• 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
• https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows

I'm on the latest Firefox (61.0.2) and running Windows 10 x64. I already found the articles you linked, but they didn't help.

Please try : SAFE MODE

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

In Firefox Safe mode these changes are effective:

• all extensions are disabled (about:addons)
• default theme is used (no persona)
• userChrome.css and userContent.css are ignored (chrome folder)
• default toolbar layout is used (file: localstore-safe.rdf)
• Javascript JIT compilers are disabled (prefs: javascript.options.*jit)
• hardware acceleration is disabled (Options > Advanced > General)
• plugins are not affected
• preferences are not affected

TEST''''is issue still there ?

I launched it in safe mode and the issue still appears. The sound starts after ~5 seconds.

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• http://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• http://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Just tried; In Safemode the same error occurs. Though I found a new symptom: The "cooldown" seems to include Firefox restarts. So if i restart firefox, sound works directly if it worked before (and then you can wait for it not to work when pausing).