Are that JSON backups or HTML backups?

You can easily merge bookmarks by importing each of the HTML backups, but restoring a JSON backup will always replace all the bookmarks. So if you have a JSON backup then you need to export existing bookmarks to an HTML file and restore them.

An alternative to merge bookmarks is to use two profiles and restore a JSON backup in each of them. Use Copy in the Library (Bookmarks Manager) of the second Firefox instance to Copy selected bookmarks to the clipboard and Paste them into the Library of the Firefox instance opened with the default profile. You need to start the second Firefox instance (profile) with the -no-remote command line switch.

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

Use the -no-remote command line switch to open another Firefox instance with its own profile and to run different Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

A user redirected to this page by the AAQ question flow was asking about this thread by PM.

Hey it is good to know the AAQ workflow is now directing to highly relevant results at least sometimes.

Other options that may be considered in some instances are

• Use Sync if you have two working computers with Firefox bookmarks
  • see How do I set up Sync on my computer?
    How do I choose what information to sync on Firefox?
    How do I set up Sync on my computer?
• Without the need to setup a second profile
  Open a second copy of the library in tab or window by using this in the address bar
  chrome://browser/content/places/places.xul
  That may help with dragging and dropping from opposite ends of a long listing. If for example HTML has just been imported in to an already large bookmarks library.