If it's a small law firm, you want to send it to one of the named partners, typically the one who has the first name listed. It's the same thing with every firm. Every firm needs to have a person that you're addressing your cover letter to. If you're not addressing your cover letter to a specific person, then that's a real problem. You just don't want to do that.
Some companies and law firms may tell you to just send it to recruiting. But you're actually better off sending it to a person. And typically, sending it to one of the more important people in the firm is crucial. One thing I want you to remember—I know I've said this in several posts—you always want to make sure that you're sending your cover letter in a printed format as well as email. You should address your cover letter to a specific person in that one, as well.