UPDATE: This appears to have been a bug in Firefox, and has been corrected in v2.0.0.5. There should *not* be a space between the selector, colon and pseudo class name. Please disregard the rest of this post 
Recently I’ve started using more CSS2 & 3 selectors and properties, including :first-child and :last-child. It’s important to remember to include the space before the colon when specifying these.
If you forget the space the css will still work, but only 90% of the time and without any consistency in Firefox 2, which of course makes it interesting trying to figure out why the page looks fine on first load but breaks after pressing F5! :-S