If you want to export your Del.icio.us bookmarks and do not care about preserving your tags and notes, then you can simply export bookmarks as HTML file. You can later import this HTML file to Firefox 4. But del.icio.us bookmarks make no sense without tags and notes, because both of them are core part of the service.
Slurp is an extension for Firefox which imports Del.icio.us bookmarks without losing any data, i.e, it can import bookmarks with tags and notes to Firefox 4.
You just need to install the extension and head over to del.icio.us website – you will be prompted with instructions. Click Yes (see screenshot), login to del.icio.us (after you are redirected to login page) and it will import the bookmarks for you.
Once imported, you can then securely sync the bookmarks across all systems using the awesome Firefox Sync and take them with you while on the go. All imported bookmarks from Del.icio.us are saved inside del.icio.us folder in your bookmarks menu.
Unfortunately, Slurp does not have a Firefox Add-ons page yet. You can download it from Gitbhub. Please keep in mind that it only works with Firefox 4.