User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0 Build ID: 20170323105023 Steps to reproduce: Open Firefox. Close the one and only currently open tab. Actual results: The tab closes, revealing a blank page with no tab icon in the tab bar. Expected results: Firefox should have closed.
Not a Firefox bug. The culprit was X-Notifier plugin.