See e.g. http://tinderbox.mozilla.org/showlog.cgi?log=Firefox/1237812361.1237819338.32480.gz *** 1348 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/toolkit/mozapps/downloads/tests/chrome/test_retention_is_0_closes.xul | Error thrown during test: this.mDownload is null - got 0, expected 1 *** 1351 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/toolkit/mozapps/downloads/tests/chrome/test_search_clearlist.xul | Error thrown during test: Cc is not defined - got 0, expected 1 *** 1354 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/toolkit/mozapps/downloads/tests/chrome/test_search_clearlist.xul | Error thrown during test: Cc is not defined - got 0, expected 1 *** 1360 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/toolkit/mozapps/downloads/tests/chrome/test_select_all.xul | Error thrown during test: Cc is not defined - got 0, expected 1 *** 1369 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/toolkit/mozapps/downloads/tests/chrome/test_unkownContentType_dialog_layout.xul | Error thrown during test: Cc is not defined - got 0, expected 1 *** 1469 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/widget/tests/test_bug478536.xul | Error thrown during test: Cc is not defined - got 0, expected 1 *** 1640 ERROR TEST-UNEXPECTED-FAIL | chrome://mochikit/content/chrome/widget/tests/test_wheeltransaction.xul | Error thrown during test: Cc is not defined - got 0, expected 1 The first one is bug 483200. Not entirely sure this is the right product/component, feel free to move it.

The reason of this failure is that the observer from test_retention_is_0_closes.xul still lingers around after that test is abruptly finished. Now that the offending test has been disabled, this should no longer be an issue. Resolving as a dupe of bug 483200.