My biggest complaint so far: they haven't made the unminified source for the JS client available! You go on npm and you get the minified source. How is that acceptable? Heck, it's completely pointless too, because if I really wanted to "steal" the source I could prettify it and clean it up. It'd take a bit of effort, but it doesn't protect or hide anything. The reason why I wanted to use firebase was so I could hack up something quick. But as I was wiring stuff up, I got confused with their docs and figured I'd take a look at the source... Only to find a minified mess.
On top of that, they're not developing the project in the open. There's no public repo, so you can't contribute. I tweeted at them earlier today; hopefully it's an oversight and they fix it.
Otherwise, it's unlikely I'll consider ever using firebase for anything, if they continue with these anti-developer practices.