There is a thread here:
How to Install and run a XulRunner Application on Mac OS X?
...in which a user mentions having trouble uninstalling XULRunner 7 so he could then install XULRunner 6. Unfortunately he just says he figured it out, but doesn't say how. It appears that I cannot private-message or email that user (CIRK).
So, does anyone know how one removes it, or at least get OS X to allow the install of XULRunner 6?
As a side-note, removing the /Library/Frameworks/XUL.framework directory does not work.
Thanks in advance.
Answering my own question a day later...
It appears that the XULRunner 7 installer not only adds files to the /Library/Frameworks folder, but also adds to /private
Specifically, I had to manually delete...
/private/var/db/receipts/org.mozilla.xulrunner.bom
/private/var/db/receipts/org.mozilla.xulrunner.plist
...along with removing the /Library/Frameworks/XUL.framework folder.
Hopefully that will be helpful to someone else.
edit: This is specifically relevant to OSX 10.6.7, reported to work on 10.7.2 as well