They both have a good implementation of various things (think [X]HTML, CSS, DOM, JS, etc, etc) which can provide both similarities (eg, Both support HTML, right?) and differences (eg, box model issues, non-XHTML-compliancy, CSS selectors, etc).

There is a huge amount that could be written on this subject so you might really want to narrow it down to one or two particular issues/points. I could write a major piece of literary work on that darned box model. Which is a tiny portion of the grand browser world.