Firefox's arguments for not really bothering with the remaining parts of Acid3 was that it seemed like it they weren't testing particularly relevant standards and that the implementations of the other browsers for those standards were mostly to pass Acid3 and not particularly useful either. See http://limi.net/articles/firefox-acid3/ for more info.

That seems like valid reasoning to me. I've also seen Hixie make comments that seem to indicate him wanting to avoid these issues for any potential Acid4:

When we do Acid4 (probably around the time we have at least three major browsers shipping Acid3-passing browsers), I think we'll have to focus on testing fewer, more critical things. Acid3 tests a lot of critical stuff, but also checks a lot of less important stuff at the same time, and it's in those areas that we've had the most problems with specs changing under us.

From http://ln.hixie.ch/?count=1&start=1215829569

I shouldn't have said 'reality', sorry. I should have said, 'relevant standards'. Acid3 used to include some deprecated standards that are no longer relevant, and has finally been updated to remove those things.