I installed another OS on my portable hard drive and didn’t want to go through the hassle of rebuilding my bookmarks. Got help from nice people and thought to share. For Firefox 3, they make it fairly simple. You only need one file – places.sqlite. Now I’m on Linux so I’m not sure if the windows version has this file or hides it. The only thing that I had to do was copy places.sqlite from my working FF3 to my new one and closed/reopened it. I got everything I need. It copies the bookmarks but not the passwords. So you’ll have to enter those again the first time that you access emails and other sites with logins.
Hope this helps!