lol broken box model
it draws margins, padding and borders differently to everything else.
you can use box model hacks though (google box model hack, i used this one:
http://www.info.com.ph/~etan/w3pantheon/style/abmh.html
(there are other hacks than these, arguments as to which are more elegant a far and wide, any of them is a cludge but this seemed a simpler cludge to me - though i'mnot sure how opera supports it)
for css/div-tastic sites I've had the most success by just using margins for layout, and then putting any padding on things like p tags.
you cant just develop for firefox though mate - cos most of the world is still using IE - and if it glitches in IE it does matter.
(something for which firefox has become a pain in the arse, when IE had like 99% market share you could ignore other browsers)
on the css front, you also have to be careful of Mac IE 5.2 and Safari cos they have their own quirks too lol