Question: When should a job offer be considered a "firm" offer for the purpose of giving notice to your current employer?
When the offer is first made, it is usually contingent upon successful conflicts, reference and background checks. The necessary information and authorizations to do those checks are usually provided when the offer is accepted.
If neither the candidate nor the firm anticipates any problems, should the candidate assume the offer is firm at this point? Or should the candidate wait to hear back from the firm before giving notice? If that's the case, how long should the candidate wait, and how many follow-up calls should he/she make?
Answer: This is a great question and one that I imagine plagues many people.
As you indicated, an offer is contingent upon references, conflicts and background checks, law school and bar verification, etc. Therefore, even though the offer is a "firm" offer when it is extended, it is not a final offer until all the potential barriers have been eliminated.
Generally speaking, when a potential employer makes an offer, they do not anticipate any problems. However, you need to be very certain that everything has been checked out before you decide to give notice at your current firm.