Thanks to your response to my review; I may delete that in a day or two since further investigation has shown my problems seem specific to a particular extension.
See attached screenshots. I was trying to install CKP, a.k.a. "Chrome KeePass", and either method for a temporary install fails in the same way. I.e. whether I click the "Install Temporarily" link or the "Temporarily Install Unsigned Addon" button the same thing happens.
What happens: The extension files download and get converted. Then all text on the installation dialog goes away. Only the buttons remain. Nothing actually gets installed.
What I expected to happen: either the extension should install, or I should get an error message.
After your reply to the review, I tried again with a different plugin. When I tried installing Save to Google Drive using the "Install Temporarily" link, the plugin installed successfully and I got a confirmation message. Oddly however, the buttons are still present in the confirmation message.
So, I can confirm this is an issue specific to the CKP extension.
If it matters, I'm running Firefox 47.0.1. I've now seen this behavior on both Ubuntu Linux and Windows 7.