BlackBerry OS 10 was my favourite one. They way it used widgets and gestures was really cool. Hub application was awesome. Android and iOS copied a lot of it later but I liked how simple and minimalistic BBOS10 was compared to them. Never tried developing for it though.
IDK, I still like them. Definitely still managing not be evil. And keep in mind they are competing with multi-billion $ corporations that pretty much control how the web works today. Google (and others of course) first turned the web into ad funded business and then used their huge ad revenue to build a really good browser and promoted it using shady practices. What was Mozilla supposed to do? Sometimes simply having better software is not enough.
People talk about FFOS like it was a failed project while in reality it was successfully commercialized and is so popular it has a native WhatsApp client. It has ~70x more users than LineageOS. Maybe Mozilla didn’t knew how to make money out of it but it’s definitely was a great OS project.