The problem is that once downloaded the plugin button to install does not light up and the installation fails, try and wait ~ 30 mini (tried it on two different computers, it’s not work too). But try to install others add-on, it’s work well.
If the installation failed in some way, the popup should show an error message. It’d be good to check the Browser Console and see if any errors or warnings pop up there.
I have the same problem, here’s what the browser says
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: Mon Nov 14 2016 16:02:49 GMT+0100 (CET)
Full Message: TypeError: is not iterable
Full Stack: AddonInstall.prototype.onStopRequest/<@resource://gre/modules/addons/XPIProvider.jsm:5974:1
Handler.prototype.process@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:940:21
this.PromiseWalker.walkerLoop@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:816:7
this.PromiseWalker.scheduleWalkerLoop/<@resource://gre/modules/Promise.jsm -> resource://gre/modules/Promise-backend.js:750:11
The only place I can find code that could cause an is not iterable exception is already a promises .catch handler.
Now it would be interesting to know why the previous call to loadManifest failed (and did not reject with an Array).
I’d suggest that somebody who encounters this problem and is interested in the solution opens The browser Toolbox (Ctrl+Shift+Alt+I (yet 4 keys)) and sets a cpuple of breakpoints in that function (resource://gre/modules/addons/XPIProvider.jsm:5666) (the line number may be slightly off).