I hate IE with a passion when it comes to CSS. 5.5, 6, 7, and coming very soon 8 all seem to render css different. 8 at least is supposed to have a standards compliant mode. Yea, we shall see.

I know there is a border line type issue that can get you up to 2 px more than you wanted in FF. say you have a box

Code
+-------+
|       |
|       |
|       |
+-------+


FF will render from within the borders the num of pixels you want.

Code
+-------+
|  ^    |
|< +    |
|       |
+-------+

arrows pointing to the inside edge of our div container



In IE, they of course like to think different, ~*cough RETARDS*~ So they render from outside the object border (even if their is no border)
Code
   +-------+
   |       |
  >|       |
   |       |
   +-------+
       ^
arrows pointing from outside into 
the inside of our div container.


This small difference can add up to 2 px difference in many layouts if not accounted for. 2px is huge. Often a work around to have cross browser compatibility is to wrap the div inside a div. Set the height width and it forces what you would expect.