1. To reproduce: go to the tinderbox page 2. Click on a 'star'. As the popup comes up, you get the assertion.
This is due to us using the wrong document (the one from nsIContent, not the one from nsINodeInfo). As a result, we get no base URI on elements that are being styled before being added to the DOM. My patch in bug 95336 fixes this. Taking; I'll resolve this once bug 95336 is checked in.
Fixed by checkin for bug 95336