Fir trees have interior needles that die after several years.   The oldest needles on the inside of the tree closest to the trunk are then cast off. The dead needles provide a valuable mulch. Needle loss in the fall is to be expected and is no reason for concern. If needles are dropping from the ends of the branches, this could be a serious problem. Fir trees produce new needles at the ends of the branches.  Mulch keeps the fir trees healthier by preventing top evaporation of moisture and regulates the soil temperature so it warms and cools slowly. A depth of 3 inches is desired.   Supplement the dead needles with woodchips mulch like to get the desired 3-inch depth. Needles that yellow and drop normally from age may have occasional spots and blemishes. Old needles sometimes show mottled brown coloration. Spots or blemishes on the current season's leaves or needles may be caused by insects or disease.
There are a few diseases of fir trees that cause the interior needles to fall off early.The following sites will help you diagnose why your interior needles are falling of early. https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/fir/needlesinteriorgray.html


https://apps.extension.umn.edu/garden/diagnose/plant/evergreen/fir/