Thanks, ʇɐb ɹəuəllıʍ
Finally, I found the reason had nothing to do with Firefox, it was because I accessed the Flash by URL:
and if I access it by my server IP(namely: 192.168.0.xxx/...html), then I can see the Flash in Firefox.
I searched the problem and it was resulted in by Flash security policy. If I added a crossdomain.xml into my webroot folder, to let it allow 'localhost' access, then I can see my flash even by URL: localhost/....