Try to move those imported bookmarks to another (new) folder.

If you can't do that either then see: http://kb.mozillazine.org/Locked_or_damaged_places.sqlite

I book mark a page and then want to add it to an existing folder which I imported from Safari. I don't seem to be able to pull the folder up to place the new book mark there.

I need help with the same problem. I imported my Internet Explorer Favourites (folders and files) to Firefox Bookmarks, where they were shown as "From Internet Explorer". Then I moved them to the Main Menu, as suggested by someone. If I go to "Organize Bookmarks" all my folders are in the library, but when I want to bookmark a page, the pulldown menu does not give me the option of adding a bookmark to one of my established folders. I select "Bookmarks Menu", then "Choose", but then I do not see my folders -- only "Unsorted Bookmarks". Is there a way in which I can add a new bookmark to the old folders directly? I shall be very grateful for an answer.