You can possibly take a screenshot of the image and save that as .png or .jpg as both can be present on the clipboard if you use Copy (you would need software that can choose either format on the clipboard).

I don't know whether this is now included in Windows 10 or was something I installed (such as HEVC video support), but I'll mention it in case anyone reading wants to check it.

When I save a .webp image, I can open it in Windows 10 Paint (right-click the image in File Explore > Open With > Paint).

There's no error message about not understanding the format, like in the old days.

When I go to Save As, Paint wants to save as .heic, the format from Apple iOS devices, but I can change it to something more typical.

Does anyone else notice this change?

That is what I did initially, but I found it easier to start using Chrome, because it does not have the limitation. If I start using Chrome for more items, I have to use it to access my CitiBank account as Firefox just locks up. I am tired of clearing the history to get in. At some pointy I will just change to Chrome and dump Firefox completely. They really have screwed up a good product.

I have the same problem with FF defaulting to downloading almost all images as .webp, which I too find of no use whatever. However, I use IrfanView as my default desktop image-viewer. And it opens .webp images. I can double-click on such an image, hit "S", and save it to .jpg format in a couple of seconds. It's a nuisance having to do that, but it's quick and easy and it doesn't require logging into another website, or writing a script, or messing around with the config, or any of that.

For anyone that doesn't have a suitable image conversion program, this add-on can convert webP and other common images to another format. It integrates into the right-click context menu, and basically uses the same technique as screenshot add-ons.

https://addons.mozilla.org/firefox/addon/webp-image-converter/