Well, no resolution but by my testing only testcase 3 failed, and that was approximately twice as better as reported when the testcase was written.
The fourth also fails. And simply improving it from failing at 14bits long to failing at 15bits isn't much of a fix at all.
Removing the overflow-y line makes no difference; the bug equally affects it with no overflow setting explicitly declared.
Granted, the forum html is a mess, but this is definitely a firefox bug. The web guys have looked into it but to work around it would basically require the entire layout to be redone again. Not exactly a fast or cheap thing to do.
I've already tried it in Minefield (FF3a8), and it fails even more spectacularly (the background stops painting before it even gets as far as 32768px).