It shouldn't happen when you're editing in a form, but maybe it's this: the backspace key. If pressed outside of a form field, it triggers the Back action.

To turn that off (like I do) and rule this out as a potential culprit, you can modify this setting:

(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.

(FYI - setting it to 1 causes the Backspace key to behave like Page Up)

Made suggested change but FF still does the same thing.

How often does this happen?

Could you test in Firefox's Safe Mode? In Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox.

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" button > Restart with Add-ons Disabled
• Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement? (More info: Diagnose Firefox issues using Troubleshoot Mode)