Hmm, that shouldn't happen. Where are the pages being saved?

For example, Firefox shows some dynamically generated lists when you click the Library button ():

• Recent Highlights -- these are selected pages from history
• (Under Bookmarks) Recently Bookmarked -- these are the most recently added bookmarks when checking across all folders
• (Under History) Recent History -- these are the most recently visited pages

But it might also be somewhere else.

they are being added to the list of bookmarks he already had.

Are they being added to the end of the "Other Bookmarks" category, and the "star" icon in the address bar turning blue while viewing the page? Other than Ctrl+d somehow being triggered on the page, it's not obvious how that could happen.

Can extensions be ruled out as a possible cause of the issue?

thanks, it will be a bit before I can get those details. the user is 85 and several states away I will arrange a time to talk to him on the phone so I can do a screen connect and watch.

I think this has to do more with a user level not understanding what those icons are and clicking on them when they don't need to. This sounds more like a user is inexperienced with the way a Browser works. It has nothing to do with Windows 10 1803 build as that is what I am using right now with FF60.