firefox doesn't display banners

Hi, I've found an identical question, with status closed. But I'm having the same problem, my drupal site with NiGraphic Studio theme looks great under Opera (windows & linux), IE7 but with firefox 3.0.3 (under windows and linux) header
banners are not displayed at all.
I'm very far from being a CSS developer, but I just tell what I did to make it work: I simply changed the filenames of the
banners (and the style.css accordingly). I've changed bg_header_banner.jpg into bd_header_banne.jpg,
bg_header_banner_1.jpg into bg_header_banne_1.jpg, etc., I simply removed the "r"
After some tests, it seems that FF doesn't like the word "bg_header_banner", even if I clear the cache; but only using this
theme: I've built a simple page with "background-image: bg_header_banner.jpg;" and it works perfectly.
Clearly, it's not a solution, but I hope it can help to find one
Sorry for bad english
Thank you
Marco