Firefox and Chrome will both save as avif by default from this site. Not checked Opera yet. I'm sure as hell not installing another browser just get images from here, I'll convert them myself in Photoshop.
Windows photo viewer though (before Win11) will not open avif or webp
by default. No idea if 11 does or not. That's why they won't open for you, it may save it as something, but the image is embedded and that's not easily changed. Try and change a webp to png etc by just changing the file extension and see what happens
Edit: See Elduriels post above.