[eluser]crikey[/eluser]
If you're talking about HTML/CSS code, I disagree... about the "non-standard code" part anyway.
I guess it depends on a few things, including DTD, complexity of the layout, special 'effects' etc. IE6's lack of support for PNG transparency using standard CSS rules, for example, can be a real pain. IE6's lack of adherence to the box model spec is likewise troublesome. Often code can be completely standard but broken in IE6.
Then there's Safari - probably worth checking your code in that too. I find Safari to be generally comparable to Firefox
Oh, and if you're *not* talking about HTML/CSS, my points are probably irrelevant.