Background image not staying fixed in Firefox

I’m probably missing something simple here, but it’s been about 5 years since I last messed around with code. My background image stays put as I scroll (as I intended) when viewing the site in IE8 and Chrome 5, but not in Firefox. I just upgraded to 3.6.3 to make sure it wasn’t a version thing before posting here. Thanks in advance for your help, folks.

My site is here.

I’m using the Atahualpa 3.4.6 theme and here is the section of code I assume is to blame:

background-image: url(/blog/wp-content/themes/atahualpa/images/backgr.gif);
background-repeat: no-repeat;
background-attachment: fixed;
background-position: center top;