The maintenance service is not checking for updates. It is still Firefox itself that checks for updates, but instead of launching the updater.exe (which triggers the UAC dialog), Firefox will start the maintenance service and tell the service there is a new update. The service will stop Firefox, run the updater to install the new version of Firefox (which does not require a UAC because the service already has elevated permissions), restart Firefox (updated version), and then stop itself. So this is not your typical update service such as Java or Adobe have which continuously runs in the background, but rather an interesting trick to run executables with elevated on-demand of a non-elevated executable.