Could you check whether this preference got changed:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box above the list, type or paste intab and pause while the list is filtered

(3) If the browser.tabs.loadBookmarksInTabs preference is bolded and has a value of true, double-click it to restore the default value of false

I checked the preference you referred to and it was already false.

That's strange. Usually bookmarks open in the current tab unless you hold down the Ctrl key or "middle click" with the mouse scroll wheel.

Are links opening in the same tab as usual?

I found the problem. I installed the extension called "Popup Bookmarks". I right clicked on the extension and selected Manage Extension. Within the extension I clicked on Options and unchecked the box "Open bookmarks in new tab". This was an extension issue and not Firefox. Thank you for your quick response in trying to help me troubleshoot this.