Firefox bug - NS_ERROR_FAILURE in nsITaskbarTabPreview.invalidate
FF 38 bug fixed in FF39: https://bugzilla.mozilla.org/show_bug.cgi?id=1127577
17:34:22.536 NS_ERROR_FAILURE: Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsITaskbarTabPreview.invalidate]1 WindowsPreviewPerTab.jsm:406:0
TBB 6.0a3 on Win 7.