By the time Firefox came out, it lined up with IE6 well enough to use the same code (W3C DOM) methods mostly. IE4.x and NN4.x support was horrific, and worse than IE6 when Google did what TFA mentioned. Dropping support for IE4/NN4 didn't mean not supporting other current browsers.
When it was in the >90% market share, a lot of people did that though.