Using Windows 7 64 bit and Nightly in a new profile.
Trying to install the TestPilot addon I get the following: The addon cannot download because of a connection failure. My Internet connection work fine, that’s why I don’t understand the motive of this issue.
I am adding the screencast for you to see
2016-08-05_1855.zip
I filed the bug: https://github.com/mozilla/testpilot/issues/1204