When speaking of famous persons, like Albert Einstein and Thomas Alva Edison, it´s fine to just use their last names. In a formal setting, as when greeting a teacher as a student, you would use the prefix Mr./Mrs./Ms. and their last name. In an informal setting like on the internet, or with people you know fairly well, it´s common to use their first names. PS It differs from country to country, and I believe that it´s more common to greet your teacher with mr. lastname in the UK, than in the US, as they generally have a more polite language.

There are probably a few million possible Alberts, as well as a few million possible Thomases. Unless a famous person is also well-known by the first name (pretty rare), then use the surname, especially when discussing a person's work. Adding "Mr/Mrs" is unnecessary unless you were addressing them directly (as Steinar has pointed out). By calling them Albert and Thomas, it sounds as if you are a personal friend of theirs, or that you want to focus on the fact they were regular people despite their fame. So using first names is possible, but there are very few situations where you could get away with doing so.

Einstein & Edison