Usually, unless it is a pinned tab, Firefox will use the same tab. Could you check the following hidden setting:

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

More info on about:config: Configuration Editor for Firefox. The moderators would like us to remind you that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.

(2) In the search box in the page, type or paste browser.urlbar.openintab and pause while the list is filtered

(3) If the preference has a value of true, double-click it (or click the Toggle button at the right end of the row) to restore the default value of false

Thank you, jscher2000. I implemented the information you provided and it worked exactly as I wanted. I'm very pleased with how it works now, as it's easy to use. Thank you for sharing the information.