I have had thick brown nasal mucus only if I blow my nose and it comes out in a string, rope like. I have never has sinus problems, not a smoker and my only symptom is a sore throat.
Brownish mucus is usually a consequence of inflammation of your sinuses. This inflammation is causing your sinuses to bleed and then this blood is mixed with regular mucus (which is clear) and you have brownish mucus discharged. This usually happens to the smokers but to nonsmokers as well.
Brownish mucus also appears when you have sore throat which is irritated. There are also various other conditions which can cause brown mucus such as certain virus or bacteria and some food (such as garlic) as well.
If you want to treat this it would be wise to buy some over the counter medication for your sinuses. If this doesn’t help it would be wise to visit your ENT doctor then. And for sore throat you can use some syrup.