You can sync virtually any setting using the [[Configuration Editor for Firefox]] (<code>about:config</code> page). You simply need to create a preference called <code>services.sync.prefs.sync.YOUR_PREFERENCE_NAME</code> and set it to <code>true</code>. To use your example of syncing the <code>network.proxy.autoconfig_url</code> preference, you would need to add <code>services.sync.prefs.sync.network.proxy.autoconfig_url</code> and set that to <code>true</code>. What some people forget is that this needs to be done on every device that you want to sync the preference to. Firefox doesn't know that you want to sync that preference, so by default it doesn't include any custom preferences that you have added to Firefox Sync. You'll also have to do this to any new devices that you add in the future. <blockquote>'''NOTE:''' I've found that if you configure your setting first and then add it to Firefox Sync, the value of the setting often gets reset. So, I think it's best to add the preference to sync by creating the preference above first and then change the setting to what you want.</blockquote> Hope this helps.