Here's a story about that firm from June of last year.
https://www.recode.net/2017/6/7/15746928/republicans-politic...
The big question for me is not whether hiring an opposition-research firm is within normal ethical limits. That ship sailed long ago, and don't think for a moment that FB is the only one who has ever hired such an outfit. The question is: why this one? Just one look at the people involved and their records would have told anyone who was paying attention that it's an exceptional sleaze magnet and engaging with them could not turn out well in the long term. Seriously, not a wise choice.