hi, what kind of security software are you using on the affected device?

I have just reread my question and realised that in my fury I failed to give complete information. My only defence is that I'm a good medical doctor, but far less competent in all things tech-related! Here goes: I am using Windows 10 64-bit OS, and have Firefox 50.1.0 installed. My laptop was bought on 27th December 2016 (just two weeks ago), and I have real-time protection from Bitdefender Antivirus Plus 2017 and Malwarebytes Pro installed. Internet connection is via high-speed BT Infinity fibre-optic Wifi (British Telecom). All drivers are up-to-date. I have checked for PUPs but found none. There is no rogue toolbar and going into the programme list via control panel shows only one modification in recent days, and that was to Firefox yesterday. I also made a mistake (unwittingly) when I said above that Spotify was not working - I've just checked, and find it is, so I apologise. The problem is with Netflix and Prime video. However, the streaming media that is unplayable in Firefox is working perfectly with MS Edge, although ever since Netscape days in the late 1990s I have preferred to use an alternative browser to the Microsoft offerings. Until now, at least - I'm ready to uninstall Firefox because I'm a busy medic, wife and mother; life is too short for this kind of hassle!!

thank you - so bitdefender probably explains it... please temporarily disable the scan "ssl feature" within that security suite like it's explained at https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER#w_bitdefender until the drm plugins get properly installed...

https://support.mozilla.org/en-US/questions/1138374

Philipp - our posts crossed! I've given extra information above. I use Bitdefender Antivirus Plus, 2017 edition and Malwarebytes Pro, just as I have always done (I transferred my licence to both when I bought my new laptop two weeks ago. Neither prevented the three missing codecs from being installed in my newly-downloaded Firefox back then.) Streaming services worked perfectly until yesterday when I refreshed Firefox, and they are still working with Microsoft Edge.

Philipp - I've disabled SSL protection, though without too much hope cos all Firefox-related addresses were whitelisted anyway. But fingers crossed!! Thank You! :-)

YES!!! Philipp, you were absolutely right! Within minutes of disabling SSL protection in Bitdefender AV+ 2017 the missing codecs were installed and I can watch streaming media to my heart's content. (My OH might not be so thrilled - my ability to binge-watch The Crown will leave him doing the dishes solo! :-)) A HUGE "Thank You!!". I'm sending the relevant screen-shots to Bitdefender - cos they might be able to prevent this happening to other people going forward, huh?

great, thanks for reporting back & glad that it is working again. bitdefender can't do much about this, as it is currently a strict security feature in firefox that will only install the plugins when there is a secure & unimpeded connection to the plugin's download server. bitdefender's scan ssl feature (and similar things that other security providers do) looks like a man-in-the middle attack to firefox... in upcoming firefox versions (52+) there will be improvements to this, so that the same problem should not occur again for you and others with a similar configuration!