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.
This pref doesn't exist by default and you may need to create a new String pref browser.bookmarks.autoExportHTML and set the value to the full path of the backup bookmarks file.
Note: an HTML backup doesn't preserve tags and annotations, so you lose those if you need to import the HTML backup.

See:

• http://kb.mozillazine.org/Export_bookmarks

Perhaps it will be helpful to others to read my notes on creating bookmarks.html:

1. I typed about:config into the Location Bar
2. I accepted the risk of changing the configuration. A long list showed.
3. I looked for browser.bookmarks.autoexportHTML. Its value was false.
4. I right-clicked and chose Toggle. Its value changed to true.
5. After exiting Firefox (perhaps the second time), I saw that D:\Documents and Settings\Chris\Application Data\Mozilla\Firefox\Profiles\xxxxxxx.default\bookmarks.html had been created. (My user name is Chris. xxxxxxxx is a string Firefox chose when I installed it.)

Thanks