I've seen this problem reported before, in the Forum.
My thoughts.....
I believe the Auctiva developers use predominanetly Microsoft tools, like Visual Studio 7.1.
Possible Firefox bug, as it's correct in IE. If it's a Firefox bug, then no problem with the Auctiva code.
Out of curiousity......
Are you doing your own picture embed or using Auctiva's image/template method?
Does it show incorrectly in preview, or only after submission on ebay?
Are you currently using an up-to-date V2 release or one of the V3 betas of Firefox?
Danno