If there are only a few bookmarks that you want to delete, you can right-click on the bookmark and select Delete.

Another option is to use the CheckPlaces add-on, you can use it to search your bookmarks for duplicates and then select the ones to delete.

The CheckPlaces add-on doesnt work well. The explanations for its different settings are not very clear. I thought it would spare the original one & delete everything below it. Instead it worked randomly & I had to look among the different duplicates to find which folder was spared & which not!!! So the program didnt work the way I thought it wil.

Fortunately there were my FF back ups. So I used this to retreive the original bookmarks with its many folders... then deleted the duplicates by hand (which costed me hours plus back- & head-aches)