You might need to check your Useragent string. You might have modified it in the past using about:config or used a Useragent Switcher addon to change it for you. You can find your UA string by going to about:support & looking under the first section called "Application Basics".

Example: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:120.0) Gecko/20100101 Firefox/120.0

Please copy it from there & share it here so we can make sure its correct.

Does it still happen in Troubleshoot Mode?

Note that macOS users on OS X 10.12/10.13/10.14 been moved to the 115 ESR update channel since your macOS X 10.14 platform no longer is supported (distributionID: mozilla-mac-eol-esr115), current releases require macOS 10.15 as a minimum.

• https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

Does it still happen in Troubleshoot Mode?

Yes, the problem persists in Troubleshoot.

You might need to check your Useragent string. You might have modified it in the past using about:config or used a Useragent Switcher addon to change it for you. You can find your UA string by going to about:support & looking under the first section called "Application Basics". Example: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:120.0) Gecko/20100101 Firefox/120.0 Please copy it from there & share it here so we can make sure its correct.

Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:109.0) Gecko/20100101 Firefox/115.0

Thanks for sharing your UA! That confirms it. And thanks to Cor-el as I had forgotten Firefox is stuck at version 115 for older OSX users.

So one workaround we can try is to use Useragent Switcher addon to trick Netflix into thinking you're using the latest version 120. Install it from https://addons.mozilla.org/en-US/firefox/addon/user-agent-string-switcher/ & you can set a custom string or preselect it from the choices they have.

You can pretend to be the latest version of Firefox or Chrome using this addon & hopefully that should allow to watch videos again.

Let us know if that is able to bypass the error messages on Netflix, Amazon Prime & Dropout.tv.

Those sites use Widevine DRM which is still supported on version 115 and uses the same plugin version as the latest version 121 (4.10.2710.0). Try unchecking "Play DRM-controlled content" in settings and then recheck it to reinstall the plugin.

Does it still happen in a new profile without signing into sync? Alternatively you could refresh your existing profile.

Refreshing the profile seems to have solved it - thanks so much, everyone!