Thats not correct. The JS files may contain code that is used by what ever the window.addEventListener fires. If it fire before parsing those JS files you will get "no such... exists"
I'am insisting that this is a bug since no one will develop the Mozilla only page. There is a "script.onload" hook to find out if the script is fully loaded but since the script is loading asynchronously the onload event should be reported before dyn. added scripts fishished loading/parsing.
And besides, where is "defer" attribute support(HTML 4.01 specification)? http://www.websiteoptimization.com/speed/tweak/defer/
The defer argument is addressed in bug 28293 Meanwhile changing the functionality right now to execute the event before the scripts load will break an unimaginable amout of webpages. Will seek further assistance from bz.
that onload waits until scripts are downloaded is not a bug. that it waits until scripts with defer attributes are downloaded is bug 28293 *** This bug has been marked as a duplicate of 28293 ***