By default the folder referenced by self.data is the "data" folder under the add-on root. According to the docs, you can change this to something else using the "data" key in package.json. So for instance an entry like this: "data": "resources" ...should make self.data be the "resources" folder. This works fine with cfx run, but with cfx xpi it seems to have no effect. Reported on the forum here: https://forums.mozilla.org/addons/viewtopic.php?f=27&t=19481 and here: https://forums.mozilla.org/addons/viewtopic.php?f=27&t=19684.
Lets just remove this property from the docs. It's probably not gonna work in jmp, anyway.
I think just removing it without any more communication/deprecation might confuse anyone who was ever trying to use it, so I added a note telling people not to use it. Let's remove it entirely when jpm replaces cfx. https://developer.mozilla.org/en-US/Add-ons/SDK/Tools/package_json#data Closing, but please reopen if that doesn't look good.