When you enable the extension (stop sign showing) the Accept header should no longer list image/webp. You can check on https://www.jeffersonscher.com/res/jstest.php When you disable the extension (yellow warning triangle is showing), the extension doesn't strip out image/webp.

If yours doesn't work either way, I can think of two possible reasons:

(1) You are using a private window but didn't enable the extension to run in private windows. In this case, the stop sign/triangle toolbar button will be missing. See the following article to fix this problem: https://support.mozilla.org/kb/extensions-private-browsing

(2) There is a conflict between this extension and another extension or proxy that also modifies the Accept header. This could be hard to track down.