Hi Irene,
It's actually a CSS problem, not a WB one, but I'll take a shot at it because I've seen it myself. There are several things that effect this, and you can try them. It's always funny to see something that works in IE and not FF, when it's nearly always the opposite.
One example is when you have "display: inline" somewhere else, and in a particular spot you're using "display: block" to override it. In this instance,
li {display: list-item}
might work better than "display: block".
Another happens when you have floated the content of your div. For instance, if you have "float: left", you may need to add more "padding-left" to your list items to get them to appear.
Based on your code, I'm guessing that the first example may help.
Good luck!