A brief case history:
When I decided to install and move to Mozilla, on my Pismo/10.2.6 (before NS 7.1 appeared upon the scene) I found to my pleasant surprise that Mozilla 1.4 simply proceeded (somehow) to go and "get" my NS 7.0.2 BookMarks, without invocation, invitation or exorhation! Being an "ancestor" has advantages for "descendants" it would appear.
However, when moving NS BookMarks to other browsers, as you say you have done, NS first requires that you transform the its bookmarks into an exportable file with an .ldif suffix.
Did you do that? I doubt that you will get the file you want if you do not do so, and if you do, everything in the file should move w/o incident. IMO.