I think I figured out what it is. I turned off web assembly in Firefox to reduce my attack surface for general web browsing (I wish I could turn off Javascript completely, but that doesn't really work these days, so NoScript is as close as I can come). I think Netflix must be the only site I actually care about that won't work without WASM, so I'm fine relegating it to a separate browser with a higher exposed surface that I never use for untrusted sites.