User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:39.0) Gecko/20100101 Firefox/39.0 Build ID: 20150325083046 Steps to reproduce: First noticed yesterday on Nightly 3-25-2015 build. Doesn't work on today's build either. Tried on both win 7 and win 8.1 64 bit, both with e10s and without. Drag XPI file from desktop onto Firefox. A new tab opens but no prompt for installing the addon is shown and addon is not installed. Install from file in about:addons does not work either. go to about:addons click install from file pick XPI in file picker Dialog closes but no prompt to install the addon is shown and addon is never installed. Actual results: Dialog asking if user wants to install the addon is not shown and addon is never installed. Expected results: Prompt should have appeared asking if user wants to install the addon, if yes addon should be installed.