In Adobe Campaign Standard, which is what forum this question was posted to:
There really isn't an out-of-box way to do this. The only way you can choose fallback behavior is with JavaScript. Here is some pseudocode which should get you started:
<% var greeting = null; if(context.profile.firstName == null) { greeting = ""; } else { greeting = context.profile.firstName; } %>
Dear Mr. <%= greeting %>
(Note that "Dear Mr." by itself does not sound natural to native English speakers, so you'd want better copy.)