it still exists, as the closed-source KaiOS. lots of feature phones use it. the first one post-shutdown was the nokia 8110 re-imagining, nicknamed the "bananaphone" because it's curved and yellow, which led to the creation of the bananahackers community.

also, since current feature phones are more powerful than the smartphones of yesteryear, it's lead to the hilarious situation that this is the most powerful firefox os phone you can buy today.