On Windows 7, if I choose Tools > Web Developer > Style Editor I get a new window. Is that the one you mean -- you want to change the setup of the Style Editor window so the list of style sheets is across the top, and the individual style sheet is on the bottom?

Hi. Thanks for the reply. I'd like to make the two windows display horizontally/beside one another, rather then top-bottom. Any idea which ID in the style sheet/style editor controls this behavior?

Unfortunately, Firefox doesn't have a built in "tile two windows side-by-side" command. There might be an add-on for that (in a quick search, I mostly found add-ons to tile two or more tabs rather than separate windows).

If that sounds completely off topic, can you describe the steps to get to what you're seeing?