It has no correlation to screen resolution. It has to do with the effective resolution of the textures used -- both image maps and othewise. Essentially, firefly resizes a texture map to match the maximum texture resolution number on the fly for a per pixel usage. This is extremely handy for highly detailed, tiled textures. The setting works best at the effective range of the texture as displayed in the render itself -- so if your render size is 1024 and you are doing a close up image of a face, you'll want to have a fairly high MTR, in order to improve the resolution of the texture. If you are dealing with a scene, and have no great closeups, then vary it according to the amount of detail necessary in the image -- but note that the higher this number, the greater the processor and memory usage will be.