The current implementation of clipPath can't handle clipPaths which have elements with different clip-rule properties or that reference other clipPaths.
Created attachment 214669 [details] SVG-code that is broken because of this bug The attached svg file works correctly in Adobe SVG Plugin (shows only red rectangle) but not in FireFox 1.5 (shows black borders around red rectangle). (The SVG code has clipPath-definition referencing another clipPath)
WFM on the trunk.
Fixed by bug 193825 (I assume).