hi, what security software is running on your device?

Windows Defender

thanks, could you try turning on logging like a firefox developer has described here at https://support.mozilla.org/en-US/questions/1135638?page=2#answer-913708 for a moment and share the results with us?

Thanks for your help philipp. Here's what the log showed. The text was rather lengthy so I've attached screenshots. Below are what look like the relevant errors:

496435438643 Toolkit.GMP ERROR GMPInstallManager.simpleCheckAndInstall Could not check for addons: {"status":"failed","results":[{"id":"gmp-widevinecdm","result":"failed"}]} Log.jsm:753 App_append resource://gre/modules/Log.jsm:753:9 Logger.prototype.log resource://gre/modules/Log.jsm:389:7 LoggerRepository.prototype.getLoggerWithMessagePrefix/proxy.log resource://gre/modules/Log.jsm:504:44 Logger.prototype.error resource://gre/modules/Log.jsm:397:5 GMPInstallManager.prototype.simpleCheckAndInstall< resource://gre/modules/GMPInstallManager.jsm:288:7 throw self-hosted:1137:9 TaskImpl_run resource://gre/modules/Task.jsm:320:42 bound TaskImpl_run self-hosted:957:17 Handler.prototype.process resource://gre/modules/Promise-backend.js:935:21 this.PromiseWalker.walkerLoop resource://gre/modules/Promise-backend.js:813:7 bound self-hosted:913:17 bound bound self-hosted:913:17 this.PromiseWalker.scheduleWalkerLoop/< resource://gre/modules/Promise-backend.js:747:11

A promise chain failed to handle a rejection. Did you forget to '.catch', or did you forget to 'return'? See https://developer.mozilla.org/Mozilla/JavaScript_code_modules/Promise.jsm/Promise

Date: Fri Jun 02 2017 16:30:59 GMT-0400 (Eastern Standard Time) Full Message: Error: page-thumbnail:error Full Stack: observe@resource://gre/modules/BackgroundPageThumbs.jsm:116:22 BackgroundPageThumbs._onCaptureOrTimeout@resource://gre/modules/BackgroundPageThumbs.jsm:305:7 Capture.prototype._done/done@resource://gre/modules/BackgroundPageThumbs.jsm:451:7 Capture.prototype._done@resource://gre/modules/BackgroundPageThumbs.jsm:471:7 Capture.prototype.notify@resource://gre/modules/BackgroundPageThumbs.jsm:428:5

 BackgroundPageThumbs.jsm:116

1496435463602 Toolkit.GMP ERROR GMPInstallManager.simpleCheckAndInstall Could not check for addons: {"status":"failed","results":[{"id":"gmp-widevinecdm","result":"failed"}]} Log.jsm:753 App_append resource://gre/modules/Log.jsm:753:9 Logger.prototype.log resource://gre/modules/Log.jsm:389:7 LoggerRepository.prototype.getLoggerWithMessagePrefix/proxy.log resource://gre/modules/Log.jsm:504:44 Logger.prototype.error resource://gre/modules/Log.jsm:397:5 GMPInstallManager.prototype.simpleCheckAndInstall< resource://gre/modules/GMPInstallManager.jsm:288:7 throw self-hosted:1137:9 TaskImpl_run resource://gre/modules/Task.jsm:320:42 bound TaskImpl_run self-hosted:957:17 Handler.prototype.process resource://gre/modules/Promise-backend.js:935:21 this.PromiseWalker.walkerLoop resource://gre/modules/Promise-backend.js:813:7 bound self-hosted:913:17 bound bound self-hosted:913:17 this.PromiseWalker.scheduleWalkerLoop/< resource://gre/modules/Promise-backend.js:747:11

I realized a pastebin link would probably be more helpful. Here ya go:

https://pastebin.com/fC8kcQcz

thanks - does a download start when you try to access this link https://redirector.gvt1.com/edgedl/widevine-cdm/903-win-x64.zip manually?

Negative, I get the attached error. I'm guessing there's something interfering with my certificate handshakes based on the content in that thread you posted but I can't figure out what it could be.

i don't think that ssl interception plays a part in your case as the error message would have looked different (referencing that a secure connection failed or the site was untrusted) and windows defender thankfully doesn't do such a thing.

it looks that something on your device is just outright blocking communication with download server for the plugin. you've already tried a bunch of things, but could you also see if setting firefox to directly connect to the internet ('no proxy') in the menu ≡ > options > advanced > network > connection - settings... is making a difference. otherwise you would have to look into other thirdparty software on your system that might interfere with webtraffic and/or also run a scan of your system with different security tools like the free version of malwarebytes & adwcleaner...

I think I figured it out. Any attempt to connect to any Google redirect (adwords, your direct WDM link, etc) results in a 404. I tested your earlier link on my phone on wifi and it refused to load. When I switched off wifi it loaded successfully. I was able to use a hotspot connection temporarily and the CDM installed.

Presumably I need to reset my router settings. I think this ticket can now be closed. Thank you for your help!