I can't seem to get any answers for this. Here's the scenario: --New install of Win7 Pro 64bit with SP1 installed -- Installed Firefox 42.0 -- Attempted to go here to install -- I click on the Add to Firefox, it shows it downloading then prompts for restart -- Addon is not installed I've tried downloading the XPI file manually and installing. Add on is not installed. I've tried syncing from a version of Firefox on another machine and what happens is Firefox is attempting to remove it. I click Undo so that I don't lose it but it's happened twice more again (I'm guessing due to periodic sync requests from the browser that refuses to install it. Needless to say, quite annoying but nothing I've been able to search for is leading me in the right direction to resolve. Anyone have any ideas?