What plugin are you talking about? If the clip played using Firefox 49, then you should already have it.

Do you have the problem with other sites also?

Here is a picture with a missing (for Firefox) plugin, many have a problem with a Firefox 49 solved with 49.0.2 and now again we have a problem with Firefox 50

https://support.mozilla.org/en-US/kb/troubleshoot-issues-with-plugins-fix-problems

https://support.mozilla.org/en-US/kb/warning-unresponsive-plugin

Remove Firefox's plugin registry file Sometimes the pluginreg.dat file becomes corrupted. You can remove it and have Firefox re-scan for plugins. As a side effect, any plugins you have disabled on the Add-ons page probably will be re-enabled, so you'll need to go back to that page one more time.

Type about:support in the address bar and press enter.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox.

Like you can see the first 3 plugins aren't installed, I follow what you say in the post but doesn't work, hoping in other help Thanks

I called for more help.

I don't know why... , but today after downgrade to Firefox 49.0.2 and update to Firefox 50, all Plugins relate to the play video start to working properly, and I can watch Netflix Thanks for the Support Cariboou

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.