Bug 728426 fixed a "leak" on a hidden browser by calling createAboutBlankContentViewer(). Social had a similar problem a while back and bug 803255 fixed this in a way that should make createAboutBlankContentViewer() unnecessary. However, recently (as in, over the last few days) the social code again started showing leaks in mochitest, and bug 944219 added a call to createAboutBlankContentViewer(), which fixed it. This implies there has been a recent regression somewhere, undoing what bug 803255 fixed.
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