This version of the plugin is out of date. If you want to get the new version you can get it here.
https://addons.mozilla.org/firefox/addon/myworld-plugin/
Thanks
myWorld Plugin Support.

Avoid this kind of useless shop-savings extensions that in facts can spy what you are looking on the internet, it's just collecting informations on you to profile you and similar customers to finally target ads/products/services. That's desguished spyware in counterpart to provide you a pretended nice service for free.

And just about the service provided: this kind of extensions (lots of similar exist like this one) can modify the webpages on-the-fly and also can censure some parts or search results to guide you to buy to a specific shop... instead of a concurrent. And guess who is paying the extension editor to do that? Sellers?
Just look at what this extension is requiring at the Permissions part...

Mind again before adding browser extensions!

not working. I tried on the latest firefox on macOS but I can't log in with the helper. nothing happens when I click login. same for safari...