It had most of the downsides of android (closed drivers, no updates after 6 months, buy a new device or get exploited, beholden to vendors, OEMs and telcos which invariably suck, central control over software distribution), and zero of the upsides (abysmal performance [even the dialer was a laggy mess], lack of software)
Except for the fact you might trust Mozilla more than google, it wasn't anymore meaningfully open than android, and suffered crippling drawbacks in comparison. Nobody except a few developers cared that you can write apps in html/js.