Extension windows opened via chrome.windows.create({type: "popup"}) will show up blank initially in the current Firefox nightlies. Right-clicking the page or resizing it makes the contents appear. Steps to reproduce: 1. Download the attached test extension. 2. Go to about:debugging 3. Click "Load Temporary Add-on" and choose the downloaded XPI file. Expected results: A new window opens with text saying "Window contents should be visible." This is what I get in Firefox 56. Actual results: The new window is blank. Right-clicking its contents or resizing makes the text appear. There are two potentially relevant error messages in Browser Console: > TypeError: invalid 'in' operand browser tabbrowser.xml:2479:1 (that's tabbrowser._insertBrowser) > Error: Missing host permission for the tab undefined I reproduced that issue in Firefox 58.0a1 nightlies (2017-11-02 and 2017-10-02) on Ubuntu 17.10. Judging by the error message, bug 1389784 might be related.