Firefox doesn't have any built-in way to create a partial export or only import part of a bookmarks.html file. You might be able to find an add-on solution for that.

You will have to edit the HTML file and remove unwanted bookmarks. To make this easier you can consider to make a copy of those bookmarks and put them at the start or end of the Bookmarks Menu or the Unsorted bookmarks folder.

You can also open the HTML file in Firefox and use extensions for removing data (usually those are for printing).

The built-in Inspector or the DOM Inspector is also able to remove content, but that might be a lot of work if a lot of bookmarks need to be removed.

• Nuke Anything Enhanced: https://addons.mozilla.org/firefox/addon/nuke-anything-enhanced/
• Remove It Permanently (RIP): https://addons.mozilla.org/firefox/addon/remove-it-permanently/