Wow. Thank you so much!
I thought I had put the #nav_bar inside the #header, but it wasn’t (which makes me wonder why it was working with any browsers!)
About the external style sheet. I am a newbie, and when I started, my goal was to make this page a template for all other pages (the dwt file was giving me trouble though, so what I did was create the other pages by saving a copy of it). Thus, I have both document-only styles and external styles. It’s a mess, and what will probably work best for me (on future sites) is to forget the template idea, create the external style sheet, and only have page-specific divs in the document-only style sheet.