Comment by abanana

Sorry for the late response. I did it in Firefox 115 ESR by setting "image.webp.enabled" to false in about:config. I just looked in there to get the exact name, using latest Firefox, and realised they've removed it. Next time I want it I'll have to look into browser extensions - looks like some don't work anymore - or stick to using the older browser for now for some sites.

If you're sure cURL is sending the Accept header correctly without WebP, e.g. just "Accept: image/png,image/jpeg" (note my error in my earlier post, it's Accept not Accepts), and the server is still serving WebP, then it's misconfigured and there may be nothing you can do, short of swearing at it.