Firefox or an add-on might be malfunctioning, but here's a more common scenario:

To see what pages Firefox has stored in the current tab's history, you can right-click the back button. If you discover more "pages" than you accessed interactively, the website may have redirected you forward to a new page and left the older page in history. This isn't always meant to trap you on the site, but that does sometimes happen. In that case, you can use that tab history list to jump backwards several pages at a time.

I don't care about going back several pages, just to go back one page. Sometimes if I click once on the back button I go back, but most of the time it takes 2 clicks to go back one page. Bruce

I was only suggesting looking at the drop-down to see whether the site had added an extra page to your history. If you have ruled that out, my next suspicion would be one of your add-ons.

A standard diagnostic to bypass interference by extensions (and some custom settings) is to try Firefox's Safe Mode.

First, I recommend backing up your Firefox settings in case something goes wrong. See Back up and restore information in Firefox profiles. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, restart Firefox in Firefox's Safe Mode (Diagnose Firefox issues using Troubleshoot Mode) using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode."

If the back button works as expected, this points to one of your extensions or custom settings as the problem.

You could check for problem plugins by also disabling those here:

orange Firefox button or classic Tools menu > Add-ons > Plugins category

Any change?

Thanks...I'll try your suggestions. Bruce