I originally couldn't figure out why this wasn't working until I opened a non-private window. It caused me a bit of a headache for being dumb and thinking I could do it in a private window on Firefox, a problem on my end more than anything.

Excellent plugin, although as someone who is not based out of the USA, it is somewhat of a limited use to me. But I can always search for the product on Amazon US to see its country of origin. Would love it if you can get this plugin to work with other Amazon sites. Also the plugin homepage in firefox store shows that it isn't being actively monitored for security by the mozilla guys. Maybe you could look into that to get it fixed.