It could be either browser settings or lack of functionality. There have been many posts and problems with use of one browser vs. another. If you search the Forum for Opera, you'll find threads on some reported problems.
Generally, Internet Explorer and Firefox seem to have the least problems and best results, as you observed with switching to Firefox. It's more a question of whether the specific browser has the functions and features to work correctly with Auctiva, than the other way around.
Danno