and there was me thinking that IE9 was supposed to be the first truly standards compatible browser.
Compatibility really has nothing to do with it. Software developers could have written software back in 1996 that would be working today if they had really thought about it and programmed it properly. Problem is they didn't think about enough. At the end of the day you can't blame failures on standards. It is human error that causes software failures since software is just human defined logic (or lack of it in this case).

having said that I think the latest human written version of javascript engine causes some problems for some human written javascript functions.

And backward compatibility has never been considered by Firefox. Just imagine if Microsoft didn't make their new operating systems backward compatible with older programs. Firefox doesn't even try. They just say "Standards" and keep changing the bloody standards. A standard which changes every other week is an exercise in futility but seems to be blindly followed by the meek. No doubt they'll inherit the world wide web. Good luck to them.