This is where it all falls apart. With Firefox 3.5, it appears as though my table must change. Firefox no longer fires onload when the Content-Disposition is set to Attachment. This is the same behavior that can be expected in Safari and Chrome. Even though it appears as though there's a growing consensus outside of the IE camp, I do not think it's moving in the right direction. (IE's behavior is clearly the best here: more, specific, expected)
Disappointedly, I'm posting this updated table depicting the events you should expect when dynamically loading iFrames:
Thanks for nothing.