That's strange. Disabling the extension should prevent it from making any changes. Any chance that the "image.webp.enabled" preference somehow got set to false in about:config? (I don't recommend setting that preference to false.)

From what I can see, 9gag uses a newer method of image substitution: a picture tag listing both webp and jpg options, and Firefox is choosing webp. This extension can't help with that issue. Currently, I recommend behind! to work with that design: https://addons.mozilla.org/firefox/addon/behind/

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.