My guess after a quick glance is the way the two interpret height

In IE if you set a height for something it's treated as a minimum height by default (if you set something to 100px tall, then put 120px of content in it will stretch).

In FF things with a height are always that height but things may overflow beyond that container in which case all bets are off. (if you make a container 100px tall then put 200 px worth of text into it, it will go past the end of the box but your background color and border, etc. will still end at 100px.)

What I usually do (and it's strictly personal preference) is something like this.

div#mycontainer {
   height:auto !important; /* ff/etc will use this because it's important. */
   height:100px; /* ie6 and under will use this because it's read last. */
   min-height:100px; /* ff/etc will use this value, ie6 and under don't support it so it will be ignored.*/
}