Firefox was actually successful, in the form of its fork KaiOS. It's a shame that Mozilla wasn't the organisation that managed to bring the OS to the people, but KaiOS is still running on millions of phones today. Mozilla partnered with them to upgrade the Gecko engine from some ancient version to a much more recent version, as well.

With KaiOS 3 not receiving many app updates necessary to continue functioning (WhatsApp, most importantly) I'm not sure how long it'll stick around, but it certainly left its mark on the mobile phone industry.