opacity < 1.0 forms a stacking context. But should z-index apply to an element that is not positioned? In the test case, opacity:0.66 and z-index:2 let a red box paint above a following blue box that has opacity:0.90 set. Not in Firefox 2.0.0.9.
Created attachment 17505 [details] testcase
See also: bug 11905.
Created attachment 21132 [details] Stop honoring z-index on objects that are not positioned.
Comment on attachment 21132 [details] Stop honoring z-index on objects that are not positioned. r=me, please add a test case (using getComputedStyle if possible, or pixel test otherwise).
Fixed in r33458.