If a form field is not marked as a password type, then Firefox may save it as part of form autofill memory.

This features can be turned off in the Options dialog:

• "3-bar" menu button (or Tools menu) > Options > Privacy panel

If the drop-down selector is not set to "Firefox will: Use custom settings for history" go ahead and select that.

Then you can uncheck the box for "Remember search and form history"

Or to edit existing entries without turning off this feature, you can use an extension such as the following:

https://addons.mozilla.org/firefox/addon/form-history-control/

Is that the explanation, or do the entries seem to be saved in some other manner?

Thank you for your swift reply. Unchecking "Remember search and form history" has not solved the problem. (I had Firefox will "use custom settings for history") and restarted Firfoox after making the suggested change.

To determine whether one of your extensions might be doing the logging in, could you test the page in Firefox's Safe Mode? That's a standard diagnostic tool to deactivate extensions and some advanced features of Firefox. More info: Use Troubleshoot Mode in Firefox.

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

Not all add-ons are disabled: Flash and other plugins still run

After Firefox shuts down, a small dialog should appear. Click "Start in Safe Mode" (not Reset).

Any difference?

Thanks. That has worked. Now I need to work out which Add-on is causing the problem.