Created attachment 8778538 [details] webext-test.zip User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:48.0) Gecko/20100101 Firefox/48.0 Build ID: 20160728204513 Steps to reproduce: My WebExtension popup contains a <div> with a "title" attribute. See attached very simple example add-on. Actual results: When opening the popup by clicking the add-on's icon and hovering over the div with the mouse, the title doesn't show up after a short wait. Expected results: The title should be shown in a tiny popup because 1) that's what's happens in common web pages 2) Chrome also shows the title. Tested with FF48 and Nightly (51.0a1).