If you substitute the appropriate prefixed property for both instances of "translate", IE9, Chrome 17 dev, and Opera Next 12.00 alpha all say "matrix(1, 0, 0, 1, 5, 0)". Firefox 12.0a1 says "matrix(1, 0, 0, 1, 5px, 0px)". Which is correct? The spec suggests the first, but it doesn't say explicitly. I suggest explicitly stating that a, b, c, d, e, and f are all numbers, not lengths.
Cf. bug 15628. Even if that bug is FIXED, I think we should still omit the "px" from all matrix entries in getComputedStyle(). Adding the "px" doesn't seem to serve any purpose -- it's shorter without "px", a matrix without "px" works in more browsers right now, and all non-Gecko browsers omit the "px" already.
Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=719446
All browsers agree now. I'll make the change when I get CVS access.
This is moot given bug 15797 comment 10. I'm leaving it open until that's resolved, though.
*** This bug has been marked as a duplicate of bug 15797 ***