Do you have any Amazon add-ons (extensions) installed in Firefox? Did you, or someone who uses your computer, install any software recently?

If it is a Firefox add-on causing the issue and you have more than one that could be the culprit, start by disabling all of them. If that fixes the issue, then it is just a matter of enabling each add-on (one at a time) to discover which one is causing the issue. When you figure out which one it is, uninstall it.