Telefónica, the major carrier in Spain, sold some Firefox OS phones years ago under the promise that there would be a version of WhatsApp for Firefox OS soon. That never happened. They eventually stopped selling them.

(Not implying there is any correlation between those two last facts.)

There is Whatsapp client Loqui IM.

They were targeting developing countries like Central Eu, South Am, Asia, rather than North Am. The market there is huge.

I live in Poland and I was lucky that I could easily find a cheap FxOS phone, which actually I am using now. This is the only truly private phone available here. It is a pity that Mozilla abandoned it. Bigger pity is that people do not give a sht about their privacy and open standards.

I live in Spain, but never saw someone using a Firefox OS phone there.