Netflix [https://help.netflix.com/en/node/32661 outlines the issue here]. They have both Mac and Windows steps listed, but since all of the steps are done in the browser, they apply to any operating system. You will have no problem following the Windows-based instructions on Linux. You will want to ensure that the '''Play DRM content''' option in Firefox is enabled and that the '''Widevine Content Decryption Module provided by Google, Inc''' plugin is set to '''Always Activate'''. If that doesn't help, try [[Troubleshoot Firefox issues using Safe Mode|running Firefox in Safe Mode]]. This will ensure that there are no extensions that are causing the issue. Finally, if you installed Firefox from your Linux distribution's package manager, try replacing it with [https://www.mozilla.org/firefox/all/ the version of Firefox provided by Mozilla]. Hope this helps.