You can make Firefox 3 create an automatic HTML backup (bookmarks.html) when you exit Firefox if you set the pref browser.bookmarks.autoExportHTML to true on the about:config page.
That backup is created by default in the profile folder as bookmarks.html, but you can set the file name and path via the pref browser.bookmarks.file on the about:config page.
Note: an HTML backup doesn't preserve tags and annotations, so you lose those if you need to import the HTML backup.