I loved Firefly. But I dropped BSG after two seasons.
I would say the look is more BSG, but the characters and story are more Firefly, although darker.
i also loved firefly. the expanse is much more noir and the crew is getting to know each other throughout the first season, so it's not really similar. i can't really say it's like firefly, but it's at least as good - as in, i can't wait for the next episode.
I'd say more like BSG, but also much better. Was never a huge BSG fan, but I'm loving the Expanse. There is much more focus one the world at large in The Expanse, compared to Firefly, and the intrigue and diplomacy between the players in that world. The rag-tag crew of do-gooders thrown together by fate in the small spaceship which half the show focuses on (and where the firefly comparison come in) are more shown as a tiny (but important) player, way out of their depth, in huge expansive conspiracy they can barely comprehend.
That sounds awesome. I loved the bit of 'world-building' that Firefly did manage to do and wished there was more of that.
I would say it's closer to Firefly than BSG, although tonally it is not that similar to Firefly. There is a definite crew-against-the-world type of vibe, but humor and pacing and focus are more militaristic and thriller based than Firefly.