You have narrowed the contenders for a piece of legal work. Each firm troops in a team to meet your staff and strut their stuff. Before that meeting, it is crucially important for the attendees from your law department to think about what they want to learn from the meeting. They need to set the agenda and stick to it. Much like vendor demonstrations of software (See my posts of Sept. 18, 2007 on several tips.), a presentation by a law firm needs to be managed.
You need to state the difficult questions and decide who will ask them. Someone needs to be sure the questions are answered sufficiently by the law firm. Someone needs to be sure all points are covered within the allotted time, and someone needs to be sure the firm gets back to the team with responses to unanswered questions. All of these steps should be addressed in a script.