This bug is similar to bug 1121698 for handling chrome windows. Marionette is only able to return a list of open tabs. But it is not clear in which state such a tab is. Especially when tests are opening or closing tabs, they have to wait until the tab has e.g. been completely loaded and transitioned (with animations turned on). This can be done by listening for the appropriate events. Currently we have this implemented in our mozmill-tests repository in the tabs module: http://hg.mozilla.org/qa/mozmill-tests/file/default/firefox/lib/tabs.js To be able to get started we need the feature to work with observers and events. This is being worked on in bug 1121691 and bug 1121702.