What side of the tree is it on. Fir are generally understory trees at least initially. If your tree is under drought stress then then needles on the south and south west sides are more likely to be affected.
Does it produce new growing tips each year? There is a disease 'fir needle cast' or rather a group of them lirulla (sic?) being one of them that infect 2nd year needles. This year's needles look fine. Last year's are infected. Spores are primarily moved by splashing. Generally trees outgrow it. I've never seen affect more than the bottom few feet.