this page is valid XHTML 1.0 Here we test / demonstrate the placement of background images in Firefox 1.5 vs. Firefox 1.0 and MSIE.
The word goes that Firefox 1.5 has
problems placing background images
at the bottom of a document.
big content, no min-height
small content, no min-height
big content, with min-height
small content, with min-height

This is how my Firefox 1.5 renders the second page, whereas Firefox 1.0 would place the image at the bottom of the viewport. I'm told that that is because FFox 1.5 adheres to the W3C CSS 2.1 specification.
The situation at the left is that the background image is at the bottom of the content, but you want it to be at the bottom of the viewport.
The perfect solution to this is to set the height of the content to at a minimum equal the height of the viewport. In other words: apply min-height:100% to the content of the document.
It so happens that this construct does not disturb renderings in FFox 1.0 nor in MSIE.

Theo van Eijndhoven iminc Jan. 2006