lol. two ways
1.bookmark->organize bookmarks, under active bookmark manager, file->export
2.copy ~/Library/application support/firefox/profiles/sdahbgcx.default/bookmarks.html to wherever u wanna export.
just an FYi there is a extension for Firefox that allows you to sync bookmarks and things with multiple computers (as long as you have a gmail account) pretty sweet, it will sync cross platform and all..
Finally... Here is an easy explanation of how to export bookmarks from Firefox to Safari. A Mac screen grab clarifies how to do it. I saved the .html file to my desktop, then imported it to Safari (Open Safari. File >> Import bookmarks then find file on desktop then add).