I gave it a spin on FF 152, macos 26.5.1
I did have the fallback emoji font work ok. When I disabled the `font-varian-emoji: emoji` declaration. I did get blanks.
According to the dev tools fonts tab, it was using Apple Emoji font. Curiously, when I had disabled font-variant-emoji, dev tools said it was using the Arr font.
The network tab said the fonts arrived as application/octet-stream instead of font/woff2. Maybe that's a place to continue.