The very simple code is on this page http://www.vuka.com/media/swf/vuka.html
All it does is run a flash movie (it does use a skin). If the skin is missing it plays fine.
If after the page has finished loading I hit refresh in Firefox it plays fine.
If I use Internet Explorer it plays fine.
If I run it under my django development server it runs fine for both Firefox and IE.
When I run it on webfaction (the production server) where the file is served up by nginx and the cache has been cleared it does not run in Firefox..
All tests done with Flash 10 (under linux and windows for Firefox). If the skin file is missing it plays (no controls) under the Firefox browser.
I checked the loading order. It does trequest the skin prior to requesting the movie, but I do not know if it finishes grabbing the skin before starting to receive the movie.
I believe the problem maybe that firefox needs to (or should) load all of the skin file before trying to load/play the movie file. The Django developmental server is probably only sending one file at a time, while the nginx tries to send both files.