The URL looks like this: https://aus5.mozilla.org/update/6/%PRODUCT%/%VERSION%/%BUILD_ID%/%BUILD_TARGET%/%LOCALE%/%CHANNEL%/%OS_VERSION%/%SYSTEM_CAPABILITIES%/%DISTRIBUTION%/%DISTRIBUTION_VERSION%/update.xml It's app.update.url in about:config The easiest way to see these values is to flip the preference app.update.log in about:config and look in the Browser Console. It will show the substitutions that are made in that URL. I have a very old post that explains some of this (the current URL are a little different) https://mike.kaply.com/2007/03/26/deploying-firefox-2-within-the-enterprise-part-5/ Information about setting up an update server is here: https://developer.mozilla.org/en-US/docs/Mozilla/Setting_up_an_update_server