>>> My Info: Win7_64, Nightly 49, 32bit, ID 20160526082509 STR_1: 1. Open url [1] in a new tab 2. Click on the page [new tab will open] 3. Start dragging selected tab 4. Drop the dragging tab to the center of content area after 2.9 seconds after Step 2 AR: A new window opens with a single tab - about:blank ER: Either no new windows, or that window should be immediately closed. Note: It takes browser ~1 second to move tab in new window, so you can try 2.5 seconds in Step 4 > [1] data:text/html,<script>onclick=function(){var W = window.open("http://example.org"); setTimeout(function(){W.close()},+I.value); }</script><input id="I" value="3000">

User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:53.0) Gecko/20100101 Firefox/53.0 I have tested this issue on Windows 10 x64 with the latest Firefox release (50.1.0) and the latest Nightly (53.0a1-20170105030229) and managed to reproduce it, however slightly different as described by the reporter. After opening the url[1], clicking on the page and waiting ~2.9 seconds after step 2, when dragging and dropping the tab in the tab area, a new (about:blank) tab is open.