1. Start a debug build from this afternoon or later.
(fresh profile optional but not required)
Among the first few lines of output is 4 consecutive instances of:
"WARNING: Serializing document without system principal: file nsXULPrototypeDocument.cpp line 424"
... followed by 2 more instances, after some other terminalspew goes by (for a total of 6)
This warning was just added today, here:
as part of bug 592943
(and the comment above the warning says that this is "worrisome"...)
Would be good to change the warning to include the URI. ;)
It turns out the warning was just wrong and we were warning when the document *had* a system principal instead of when it doesn't.
This bug was resolved by the backout, and shouldn't come back since I have a fix for the warning in the original bug. I do have a patch to include the URI, but the warning doesn't appear anymore so I don't care so much about pushing for it.