diff -r 000000000000 -r 6474c204b198 dom/browser-element/mochitest/priority/CAUTION
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/browser-element/mochitest/priority/CAUTION	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,15 @@
+A word to the wise:
+
+You must ensure that if your test finishes successfully, no processes have
+priority FOREGROUND_HIGH.
+
+If you don't do this, expect to see tests randomly fail with mysterious
+FOREGROUND --> FOREGROUND priority transitions.
+
+What's happening in this case is that your FOREGROUND_HIGH process lives until
+the beginning of the next test.  This causes the process started by the next
+test to have low CPU priority.  Then the FOREGROUND_HIGH process dies, because
+its iframe gets GC'ed, and we transition the new test's process from low CPU
+priority to regular CPU priority.
+
+Ouch.