I've read around the Web that the extension manager can be asked to
check if an extension is about to be uninstalled (something about a
'toBeUninstalled' flag). Could anyone explain how to do so? I can't
manage to access that flag.
This is very old, but I think it still works:
http://xulsolutions.blogspot.com/2006/07/creating-uninstall-script-for.html
- Jorge Villalobos