Getting a lot of these issues lately, but give this a try. I am able to replicate the issue.

https://addons.mozilla.org/en-US/firefox/addon/content-type-fixer/

Add-on installed and showing on your toolbar. Click it to insure that the hammer is not showing with zzzzzzs and then click on the Add/ Edit option. This will open tab with a button to Refresh Log. Then go back to the tab with the site and refresh it, right click on the image and then save as and then close the save as window. Go back to the tab with the Refresh Log button and click it. Then you will see the option to add it.

wow... yeah the add-on needs better instructions.

That's a question for the Dev team.

As a possible workaround you can inspect this image request in the Network Monitor and use Edit and Resend to request the image a modified request header that doesn't include image/webp.

• Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
• Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

You can save this image via the right-click context menu in the left panel (Save Image As) of that new request that should have: content-type: image/jpeg

You can look at an extension like one of these.

• Don't "Accept" image/webp https://addons.mozilla.org/firefox/addon/dont-accept-webp/
• WebP image converter https://addons.mozilla.org/firefox/addon/webp-image-converter/
• Save webP as PNG or JPEG https://addons.mozilla.org/firefox/addon/save-webp-as-png-or-jpeg/

minger, the image is NOT A WEBP, yet firefox saves it as such. if none of you will help me prevent firefox from ruining my life, you will tell me how to contact a firefox employee so that i can deal with someone who understands how to solve an incredibly simple problem

That the link request includes 8i4320q0p9941.jpg doesn't mean that much. If the server thinks that the browser supports WebP images then you will get a WebP image and not a JPEG. That is how it works these days. In this case the link also include '&auto=webp' and that probably causes the server to default to WebP.

• https://preview.redd.it/8i4320q0p9941.jpg?width=960&crop=smart&auto=webp&s=47ae6d7628e439b5f744cd13802b96635106ba08

As you can see, it is still possible to force the server to send a JPEG image in Firefox 84 with some effort.

You can see the result from both requests in this screenshot. first the image send by the server where the request includes image/webp and a WebP image is send and the second request where the image/webp is omitted and a JPEG image is send.