Its big thing originally was being a 64-bit port of Firefox. I assume it's still based on Firefox in some way, based on the name, but not really sure what it brings to the table anymore, besides some minor stuff you can already get with extensions.
Its thing nowadays is being a fork of a Firefox release old enough to still support XUL. Which means it's effectively a 4+ year old version of Firefox, with no renderer isolation and no support for most web features added in the last four years.
