You will have to edit this HTML backup and remove the unwanted bookmarks from a copy of the HTML file.

My Q was: will importing a couple of bookmarks overwrite the existing bookmarks on the pc? I don't want to lose any of the existing bookmarks?

thanks

The Import from HTML process should add rather than replace (it causes duplicates). But just in case: you can make a JSON-format backup first. If you end up restoring this backup, it will roll back your bookmarks to the set in the backup. See: Restore bookmarks from backup or move them to another computer.

What I can't tell you is exactly where the imported bookmarks will end up. Traditionally, when you imported a file created by IE/Chrome/Edge, it was common to find them all in an Imported Bookmarks folder at the bottom of the Bookmarks Menu. However, there have been some tweaks to this process over the years, so it's possible that Firefox will try to add them to their original locations rather than grouping them all together.