Both as a reference and as a form of address, Dr. Lastname is the most common format, as is the case with most titles in English-speaking countries (Mrs. Robinson, Cardinal Naguib, Private Benjamin, Lord Snow). But Dr. Firstname is not unheard of, and depending on the relationship you have with the doctor and the setting for your interaction (including the geographic part of the world), may be unremarkable.
Anecdotally, I would expect Title Firstname to be more common in environments where social hierarchy is stricter, and to be more common for more exclusive titles. For example, in much of the Southern US, which is more socially conservative than the country as a whole, children might address a familiar adult as Mr. Firstname or Miss/Mrs. Firstname to be friendly but respectful. I also understand this to be practiced in India and other parts of Asia. It is not at all common where I live now, in the Washington, D.C. area, nor where I grew up, in Southern California.
But in both SoCal and DC, people certainly do follow this pattern for loftier titles. At my elementary school, we addressed the chaplain as Father Patty, not Father O'Reilly (or for that matter, Father Patrick). In my brief stint as a lowly editorial assistant at the National Research Council, despite a surprisingly collegial atmosphere, I still referred to the researchers in my office as Dr. Judy or Professor Jake instead of simply Judy or Jake (too familiar), or as Dr. Chen or Professor Richardson (too distancing).
Similarly, staff in a medical office might address the doctors as Doctor Jim and Doctor Maggy even though the patients invariably address them as Doctor D'Angelo and Doctor Singh.
Address your doctor whichever way she or he prefers: that is the long and short of it. Closely related questions include Can "Mr", "Mrs", etc. be used with a first name? and First name or last name with "Sir" ; also see Disrespectful use of "mister" and How should I address a professor in the US?