You can test using https://i2.wp.com/radical-mag.com/wp-content/uploads/2019/08/@Test-BMW-320d-8.jpg -- normally, if you hover your mouse over the tab, it will say it is a WEBP image; with the extension enabled, it should be a JPG image.

If it doesn't work on that one, check whether it's the private window issue: https://support.mozilla.org/kb/extensions-private-browsing

If you want to send me some problem URLs, you can create an issue at https://github.com/jscher2000/dont-accept-webp/issues or email me at jscher2000@outlook.com.