I have two sets of Firefox bookmarks: Set A is used at home and has over 500 entries. Set B is used at work and is a subset of set A (does not include gaming, sports, hobby sites etc). When I add a link to the "common area" (folders shared by both sets) I can't use any of the existing sync tools because they will sync everything in both folders, effectively creating two copies of Set A. So I have to log in and out of Firefox with different profiles for the two sets of bookmarks, copying and pasting links one at a time, and hoping I remember to get them all matched up.
If I could export Set A to homemarks.html and Set B to workmarks.html and then look at them in a two-pane window, dragging bookmarks back and forth, I could then re-import the files to their respective Firefox installs.