This can be caused by installing an extension. Since your "More system details" does not list your extensions, could you go to the following tab and disable ALL nonessential or unrecognized extensions. Don't be fooled by bland names like Browser Manager. If in doubt, disable:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

A restart link should appear above a disabled extension in most cases, so use that to restart Firefox and see whether the problem goes away. If it does, you know the culprit is one of those disabled extensions.

That seems to have done the trick. Thank you very much