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.

In case you are using private windows, make sure you enabled the add-on to run in private windows: https://support.mozilla.org/kb/extensions-private-browsing

If that's not the problem, hmm, could there be a conflict with a different extension that also modifies request headers?

What this add-on does is remove a signal that Firefox normally sends to the server that your browser can handle WebP images. The idea is to discourage the server from sending WebP in place of JPEG or PNG.

I'm not sure what problem you ran into with saving an image not working, but do you mean you still got a WebP image? That definitely can happen when the site insists on sending you one, or when the page is using a "picture" tag, but this add-on should help on most sites.

If you need an add-on to convert a WebP image to something else, check out https://addons.mozilla.org/firefox/addon/save-webp-as-png-or-jpeg/