Can you reproduce this issue in a troubleshoot mode?

Make sure you are not blocking content.

https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop also see https://blog.mozilla.org/security/2021/03/23/introducing-smartblock/

https://support.mozilla.org/en-US/kb/smartblock-enhanced-tracking-protection

Diagnose Firefox issues using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Many site issues can be caused by corrupt cookies or cache.

Warning ! ! This will log you out of sites you're logged in to. You may also lose any settings for that website.

• Delete cookies to remove the information websites have stored on your computer {web link}

• How to clear the Firefox cache {web link}

I cannot reproduce the issue in troubleshoot mode

It could be the work of one of your add-ons, or it could be Hardware Acceleration. How to disable Hardware Acceleration {web link}

Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Disable a few add-ons, then Restart Firefox.

Some added toolbar and anti-virus add-ons are known to cause Firefox issues. Disable All Of them.

If the problem continues, disable some more (restarting FF). Continue until the problem is gone. After, you know what group is causing the issue. Re-enable the last group ONE AT A TIME (restarting FF) until the problem returns.

Once you think you found the problem, disable that and re-enable all the others, then restart again. Let us know who the suspect is, detective.

You can check the Web Console for media related messages.

• "3-bar" menu button -> Web Developer or Tools -> Browser Tools -> Web Developer Tools
• https://developer.mozilla.org/en-US/docs/Tools/Web_Console

I fixed it by disabling the adblock add on, saw it was preventing the player from starting

That was very good work. Well Done. Please flag your last post as Solved Problem as this can help others with similar problems. Go to that post and click the 'Solved' button to its right.

Load the web page. After the page is loaded, click the AdBlock icon. Now select Disable on <website>. After, reload the page.

Chosen solution is correct, but be sure to look at the Network tab and refresh the page. It should say what blocked the particular resources. In my case, I'd forgotten about DuckDuckGo Privacy Essentials after already disabling uBlock Origin and Privacy Badger. Only DDG was breaking the site, not the other two.