Is this problem a recent development or has Firefox always done this?

If it's fairly new, can you associate it with any software changes, such as an update to Firefox or your add-ons, or an update to Windows, or an update to your mouse-related software?

Firefox has a built-in shortcut for the Backspace key returning to the previous page. For a mouses/trackball with extra buttons, one of them often is configured as a Back button. Shouldn't be the right button, but... As a diagnostic, could you disable that and see whether it makes any difference? Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

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

(3) Double-click the browser.backspace_action preference and change the value from 0 to 2 (or anything other than 0 or 1) and then click OK.

Any difference?

I noticed this today, I seem to recall yesterday getting a FF update. And yes it worked! Thank you so much!!!! One piece of my sanity restored. :)

So it started happening again. I repeated what you had me do above and the value was set to 2 which is what I changed it to yesterday. I now changed the value to 3 and it stopped again, but concerned that I'm going to have to do this everyday.  :/

Ok so it did stop for a minute but now it's doing the same thing all over again.

It must be something else, but I'm having trouble thinking of how right-click could get wired to Back.

Is it a standard two-button mouse with a wheel in between?

The first item in the right-click context menu on a web page is Back.

Can you try a different mouse in case there is an hardware problem with the mouse?

If is is a wireless mouse with a battery then try to replace that battery.