hi Majr, you'll probably have to get in contact with netflix' support about this issue since we cannot control the requirements they implement on their side... thank you for your understanding!

You can try to clear the Netflix cookies.

Does the HTML5 media player work otherwise without issues (e.g. YouTube)?

Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

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

cor-el said

You can try to clear the Netflix cookies. Does the HTML5 media player work otherwise without issues (e.g. YouTube)? Start Firefox in Safe Mode to check if one of the extensions (Firefox menu button/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox menu button/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

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

I said I was using a fresh profile, so there is no cookies to clear, and no extensions to disable. Still doesn't work in safe mode.

HTML5 works fine on YT (MSE enabled), HTML5test reports that DRM is enabled.