xulrunner
is just a runtime or platform. You'd need an actual app on top of that. In that regard, Firefox, Thunderbird, Seamonkey, InstantBird, etc. can be considered (complex) xulrunner
apps.
Existing extensions may run, with or without modifications in such a xulrunner
app. Well, likely with modifications, as most extensions use APIs specific to Firefox or Thunderbird or ...
The add-on SDK is not generally isn't a good fit for generic xulrunner
apps: It is too much tailered against Firefox specifically.