Answering my question in a day ...
It seems that the XULRunner 7 installer not only adds files to the / Library / Frameworks folder, but also adds to / private
In particular, I had to manually delete ...
/private/var/db/receipts/org.mozilla.xulrunner.bom /private/var/db/receipts/org.mozilla.xulrunner.plist
... along with deleting the /Library/Frameworks/XUL.framework folder.
Hope this will be helpful to someone else.
edit: This is especially true for OSX 10.6.7, reportedly for working with 10.7.2, and also
user550082
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