There is an addon that helps to manage duplicates that may be helpful to you

• Bookmark Deduplicator 1.3.1.1-signed
  https://addons.mozilla.org/firefox/addon/bookmark-deduplicator

I suppose the other method is to try to export the Google Chrome bookmarks as HTML then edit and import that file. Many word processors will be able to edit HTML file. But you would need to take care to leave it in the HTML format without adding any other formatting or changing the file extension. (The sometimes hidden HTML ending)

• Import Bookmarks from an HTML file

A variant of that method, if there are only a few new bookmarks, would be to take advantage of the fact that Firefox should be able to open a html file directly. You would be able to see all the bookmarks, and expect then to be clickable links. You could individually open and bookmark some of those links. The Firefox keyboard shortcut Ctrl + O allows you to navigate to and open files.

Thanks John99. I have had a quick look at that addon and it looks like it will do the job. I'll give it a try and let you know how I get on.

I don't know whether you saved passwords in Chrome that you want transferred to Firefox, but there is a different feature for that: Import data from another browser.

I suggest creating an Export of your existing Firefox bookmarks before using that one. This article has the steps: Export Firefox bookmarks to an HTML file to back up or transfer bookmarks.

Thanks jscher2000 as well, and many thanks to both of you for very prompt and helpful replies. I wish that some of the other fora that I use were as good.

I have used the deduplicator after doing a backup, and keeping interim ones as well. It is a bit aggressive but has done the job.

Once again many thanks.