UPDATE: Added an exemption for Patreon (patreon.com, www.patreon.com) in version 0.8. Hope that works for you.

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Thank you for the suggestion. I looked at how to do it and there is are a lot of user interface bits to build out, so it's going to take a little while.

Meanwhile, I don't know why a website would be concerned about the changes to the Accept header -- perhaps they look suspicious when image/webp and/or image/avif is removed? If you hear anything from Patreon about it, please let me know.

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.