Actually, it has been recently changed to where during boot if an extension has a startup script, that startup script name is echoed to a file and the order of extensions is loaded alphabetically, but with dependencies loaded first if a dep file exists for the extension. At the end of boot, the name of the startup scripts exist in the file in the order that the extensions were loaded. Then at the end of boot after depmod and ldconfig are run, the startup scripts are executed from that list.
In other words, if dbus has expat listed as a dependency, then expat gets loaded and it's startup script is run before dbus's both during boot as well as with appbrowser use.