this can be handy(if not slightly irritating) for navbars.
the following site uses a fixed image background which is not a very good thing in my opinion because it's sluggish and uses quite a bit of bandwidth so load times are far from good.
There exist both good and bad traits of fixed positioning. The good is that it keeps the navigation present at all times on the page and it creates an interested effect on the page. The bad is that there are some usability concerns..
I found that a couple of styling setups didnt work in IE as I intended. Chrome seemed to pick up the designs better I found.
But as you said the difference in the browsers are frustrating.