Mozilla could could use more unit tests for content lists. See the comments in the test file.
Specifically, what we need are ways to test that content lists respond correctly to document mutations. We need this for both content lists whose root node is in a document and ones whose root node is not in a document, since on the 1.8 Gecko branch the two cases behave differently.
If someone out there is willing to write some tests, I can make sure they get into CVS in an expeditious manner. We have a crash-fix-thingie that we really need to test well before we try landing it on branches...