I was in same situation and wrote script to solve it.
You need to following steps, if you want to use it.
0. install ruby environment include library(rubygems, rexml, json) and svn client environment.
1. svn checkout http://svn.assembla.com/svn/import_delicious_bookmark_into_firefox3
2. backup firefox 3 bookmark.
[Bookmark] -> [manage this bookmark] -> [import and backup] -> [backup]
3. export del.icio.us bookmark.
access
https://api.del.icio.us/v1/posts/all and save it.
4. run script
./import_delicious_bookmark.rb <created file at 2> <created file at 3> <output_file>
e.g.
% ./import_delicious_bookmark.rb bookmark_2008-xx-xx.json all.xml imported_bookmark.json
5. restore firefox 3 bookmark by using step 4's file.
[Bookmark] -> [manage this bookmark] -> [import and backup] -> [select file] -> <created file at 4>
enjoy!