Or at least, Firefox v119.0 can display WebP and AVIF favicons, so that’s something. I’ve opened an issue on the ‘caniuse’ repo for tracking overall browser support, so if you’re willing to do some testing with your browser, that’d be much appreciated.
It’d be especially useful if you test old browsers to hopefully find definitive cut-off points for each browser version.
Oh yeah, also, Happy Halloween!