I just found out that MozBug 24998 (background images spill beyond rounded borders / -moz-border-radius) was fixed late last year. Nice!
Here’s a screenshot of a quick demo - it uses HTML but the same can be done in XUL. See how the corners of the background image don’t show through the edges of the porthole like before?
Songbird: Songbirdnest.com blogs
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