We have a couple of Mn/MnH, and Wd failures which are related to this particular problem, and which I want to all make dependent on this bug. Basically what happens here is that a page load is triggered via Marionette, and which is successfully finished. But at this time Firefox doesn't show any content in the tab (see attached screenshot). It's just a blank page. Also retrieving elements fail because they cannot be found in the DOM.

With the fix on bug 1504807 it's now clear that we do not end-up at the requested URL but on `about:newtab`. Here a log snippet from a failure for bug 1503027: https://treeherder.mozilla.org/logviewer.html#?job_id=210228135&repo=autoland&lineNumber=27192-27204 > 1541556831121 Marionette TRACE 96 -> [0,18,"WebDriver:Navigate",{"url":"data:text/html;charset=utf-8,%0A%20%20%20%20%20%20%20%20%20%20%3Cinput%20type%3D%22text%22 ... B%0A%20%20%20%20%20%20%20%20%20%20%20%20%7D%29%3B%0A%20%20%20%20%20%20%20%20%20%20%3C/script%3E%0A%20%20%20%20%20%20%20%20"}] > 1541556831153 Marionette DEBUG [8589934605] Frame script loaded > 1541556831163 Marionette DEBUG [8589934605] Frame script registered > 1541556831201 Marionette DEBUG [8589934605] Frame script loaded > 1541556831202 Marionette DEBUG [8589934605] Frame script registered > 1541556831299 Marionette DEBUG [8589934608] Frame script loaded > 1541556831318 Marionette DEBUG [8589934608] Frame script registered > 1541556831334 Marionette DEBUG [8589934605] Received DOM event beforeunload for about:newtab > 1541556831338 Marionette DEBUG [8589934605] Check readyState complete for about:newtab > 1541556831347 Marionette TRACE 96 <- [1,18,null,{"value":null}] > 1541556831364 Marionette TRACE 96 -> [0,19,"WebDriver:FindElement",{"using":"tag name","value":"input"}] > 1541556831367 Marionette DEBUG [8589934605] Check readyState complete for about:newtab > 1541556831421 Marionette TRACE 96 <- [1,19,{"error":"no such element","message":"Unable to locate element: input","stacktrace":"WebDriverError@chrome://marionette ... :532:3\nregisterSelf@chrome://marionette/content/listener.js:460:5\n@chrome://marionette/content/listener.js:1682:1\n"},null] The problem is actually here: https://searchfox.org/mozilla-central/source/testing/marionette/listener.js#149-161 After a frame script has been moved to a different content process, and registered itself again, we check for the `readyState` of the document to not unnecessarily register page load event listeners. Hereby `handleReadyState()` gets the `documentURI` which in this case is `about:newtab`. Given the magic of that page, it might have been loaded by Firefox before loading the actual page. The latter we don't recognize anymore because `readyState=complete` causes the command to return. I wonder if this is actually a problem with our `open_tab()` helper of the `WindowManagerMixin` class. It doesn't wait for the initial page (in this case about:newtab) of the new tab to be loaded, but immediately triggers a navigation request. Given the upcoming implementation for bug 1504756 the mixin class might not longer be necessary. So lets depend on it.

The problem as reported on this bug should be the affect of loading `about:newtab` when opening a new tab. So far I thought that we don't do that, but bug 1506643 proved otherwise. With the landing of the patch on that other bug, we no longer use that about page but `about:blank`. As such this bug should no longer be present anymore now.