Okay, now it works. If you've got the same or similar problem, edit harness-options.json file. After
"page-mod": {
"path": "addon-kit/lib/page-mod.js"
},
add:
"preferences-service": {
"path": "api-utils/lib/preferences-service.js"
}, /*with or without the comma, as the case may */
That's all :)