If you exact-search that message on google, it appears to be generated by Cloudflare's challenge.

I found someone complaining about a similar thing except on WhatsApp Web which also tells them to update Firefox even though they're on FF 107.

Maybe FF changed something used in common feature-detection checks. Perhaps it's fixed in FF 108+.

it actually links to this https://developers.cloudflare.com/fundamentals/get-started/c...

I think this is probably something that only cloudflare can fix

I'd guess it's just the sad-but-inevitable result of Firefox market share dropping so far. Remember back when every website would tell you your browser was "out of date" unless your user-agent matched IE6's?

For everyone not on Firefox, this is one reason why we kept saying a browser monoculture is bad.

I don't think it's inevitable, there a lot of small vocal groups that achieve their goals ...