Firebase is a young startup - the company was founded in 2012 - that was recently acquired by Google. The acquisition size was not disclosed, but is almost definitely in the tens of millions of dollars.
Firebase does not have any patented amazing technologies that google had to own. Also, they have just finished a funding round one year earlier - so the startup definitely did not need the money and wasn't desperate for an acquisition. So what made google so eager to make the deal?