The CSS specification states that once element is taken out of normal flow by floating left or right, it no longer contributes to the size of it's container.
This first example contains a UL with all its LI floated. See how the 2 px border collapses to form a single dashed line 4px thick. The floated contents have no effect. This is correct.
However, if viewed in IE this example will actually have the dashed border form a proper box around all it's contents. Even though this behaviour is wrong, it's how IE behaves all the time and so people have come to expect and even depend on IE acting like this. I wasn't able to reproduce it here but I've seen in a few of my layouts I could somehow get IE to display a collapsed border as if it were actually following specifications (the block element was still expanded but the border displayed properly collapsed).
Example #2: Here we have a fieldset containing the same UL with floated LI.
In Firefox it's obviously auto expanding to surround it's floated children.
This is exactly the same behaviour as IE!
This is Wrong!....but Safari gets it right
And while we're on the topic of Fieldsets. Why can't browser makes decide once and for all how the Legend interacts with elements around it and how it can be styled consistently?
I've specified the margin and padding of the legend and fieldset on the following and they look far too different in IE and FF to be acceptable. And, yes, this Photo fieldset has the same problem in Firefox as described in Example 2.