As I have answered previously, I am not using Firefox in private browsing mode, nor do I have Firefox set to never remember history. I am not going to argue with you over my fresh profile.

For ease of reference:

New Profile Test

This takes about 3 minutes, plus the time to test your problem site(s).

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Click the Create a New Profile button, then click Next. Assign a name like Dec2018, ignore the option to relocate the profile folder, and click the Finish button.

After creating the profile, scroll down to it and click the Set as default profile button below that profile, then scroll back up and click the Restart normally button. (There are some other buttons, but please ignore them.)

Firefox should exit and then start up using the new profile, which will just look brand new. Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system so we can get a clean test.

Do your problem site(s) work any better in the new profile?

Firefox will need to download and install Widevine in the profile.

When you are done with the experiment, open the about:profiles page again, click the Set as default profile button for your normal profile, then click the Restart normally button to get back to it.

OK OK jscher2000 You badgered me into creating a new profile for the third time. I followed your instructions in the previous post EXACTLY. The result is the same Netflix error message that I always get: Error code: F7121-1331. You should have believed me that it was a waste of my time. Now do you have any real help to offer? Has Mozilla made any effort to find what percentage of their users have this same problem? Netflix says that it is a known Firefox issue.

Hi loyall, thank you for testing. We can rule out a setting stored in your profile as the problem.

I don't have Netflix, so providing the steps for that test was all I planned to add.

OH great . You jump into this thread after five months, insist that I create a new profile for the third time, and then bow out without offering any further help. Nice. Why does Mozilla refuse to take up this matter? Should I just assume the Firefox Quantum browser is toast?

Hi loyall, I didn't insist on anything. You'll notice my reply starts with "For ease of reference" because there were many posts about whether to create a new profile but the steps weren't listed anywhere as far as I could tell.

Boot the computer in Windows Safe Mode with network support (press F8 on the boot screen or hold down the Shift key) to see if that has effect.

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

cor-el, I did as you instructed and started Win 10 in safe mode. Then when I tried to stream a video on Netflix, I received a message that Microsoft Silverlight is required. So I installed Silverlight, but when I tried once more to stream the Netflix video I received the message again to install Silverlight. What do you make of this??

Hi loyall, Firefox 52+ no longer run Silverlight; Netflix should use the built-in HTML5 player + Widevine.

However, you mentioned on page 1 of this thread that you've already checked your Firefox's HTML5 support and did not find any problems. That said, given this new behavior, you could re-check:

• https://www.youtube.com/html5/ - all boxes have a blue checkmark
• Fix video and audio problems on Firefox for Windows N editions

With respect jscher2000, I'll wait for cor-el to respond. Netflix now seems to be requiring Silverlight. Let us see what cor-el says about this. I have been waiting five months for a solution.

Like posted above by Jefferson, Netflix should use the HTML5 media player and the Widevine plugin to play DRM protected media in Firefox.

Firefox should show the DRM icon at the left end of the location bar when a website tries to play protected media.

See also this Netflix troubleshooting page.

• https://help.netflix.com/en/node/32661

Does the Widevine plugin work here:

• https://shaka-player-demo.appspot.com/

cor-el, 1) My Firefox shows the DRM icon at the left of the location bar. 2) I have taken all the steps for Windows shown in your first link. 3) The video plays correctly on the Google Shaka player in your second link.

loyall said

Netflix now seems to be requiring Silverlight.

Which would be silly since Silverlight support for IE7-8 was removed between 2014-2016, depending on the OS and Silverlight only supports IE9-11 will last until late 2021. There is no Silverlight Plugin available for Microsoft Edge.

Chromium, Chrome, Opera dropped support for NPAPI Plugins completely back in Sept 2015. Firefox has not allowed Silverlight to run since the 52.0.x Release but did in the obsolete EOL Firefox 52 ESR (52.9.0esr).

All I know is that cor-el instructed me to boot Win 10 into safe mode and then try to stream Netflix on Firefox. I got a message to install Silverlight, so I did. So don't call me silly!

Atten: cor-el, Here is a screenshot in Win 10 safe mode of Firefox asking me to install MS Silverlight. It is my understanding that Firefox no longer supports Silverlight. What do you make of this screenshot? What should I do? Silverlight remains installed on my computer, but it is not listed in FF plugins.

Maybe some Windows components that are required to run Widevine properly aren't available in Windows Safe Mode and that makes Netflix fallback to Silverlight.