Push preferences to b2g device

gistfile1.md

Create a file called user.js:

pref('toolkit.identity.debug', true);
pref('toolkit.identity.uri', 'https://login.dev.anosrep.org'); // for example

Push it to the device:

adb shell stop b2g
adb push user.js /data/local/user.js
adb shell start b2g

If it's already there, you can adb pull /data/local/user.js, edit it, and push it back.