User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:46.0) Gecko/20100101 Firefox/46.0 Build ID: 20160502172042 Steps to reproduce: Some time ago, I rewrote my Firefox (XUL/XPCOM) Tile Tabs add-on as a Chrome extension. In the Firefox (XUL/XPCOM) version, multiple tabs are arranged in a tiled layout within a single browser window. In the Chrome version, because of the restrictions of the Chrome API, multiple tabs are arranged in a tiled layout of multiple windows (one tab per window). I am now rewriting the Chrome version to create a single generic WebExtensions version of Tile Tabs that will work with both Firefox and Chrome. Because Tile Tabs does a lot of window creation operations, it is very obvious that chrome.windows.create is much slower in Firefox than in Chrome. The difference in window creation sppeed may be due to the underlying technologies used to implement Firefox and Chrome. However, I think the user perception is exacerbated by the fact that newly created Firefox windows appear to grow quite slowly to their final size in comparison to Chrome. Is it possible to grow newly created Firefox windows more quickly to their final size. If so, perhaps this change should only apply to windows created programatically be add-ons.