I am using the Firefox Add-on SDK to create an extension and am performing a PageMod. This code is in main.js.
...
exports.main = function() {
I think you need to use absolute paths in those url fields. So figure out what seld.data.url('fonts') (by like console.log'ing it) (the path should be something like resource://weraweraer-at-jetpack/data/fonts) and then update your url's to use that path.
I'm pretty sure this should work because resource url's don't have security restrictions. See this page here: https://developer.mozilla.org/en-US/docs/Chrome_Registration#resource
It says:
Note: There are no security restrictions preventing web content from including content at resource: URIs, so take care what you make visible there.