The browning on the arborvitae looks like it may be due to a possible root or trunk problem. This can be difficult to diagnose. Arborvitae can remain green for a while before they turn brown.
Look around the base of the trunks or stems for rabbit (gnawing around the base of trunk) or vole damage. Look for silver dollar sized holes in the soil around the base of the plant. Voles can feed on the roots of trees and shrubs and cause dieback. http://extension.umd.edu/hgic/voles-groundcovers
Look along the trunk for small insect holes on the arborvitae. There is a new invasive pest called the japanese cedar long horned beetle that can cause branch dieback and browning. Look carefully for oval exit holes about 4mm x 2mm on the damaged limbs. Peel back bark and look for tunnels. http://extension.umd.edu/hgic/invasives/japanese-cedar-longhorned-beetle
Also, check the drainage in the area and make sure the soil drains well and there are no drainage spouts dumping water. Mulch should be no thicker than two inches and keep away from the base of the stems.
All you can do is prune dead branches and look for the aaove issues. The pruning will leave big holes and the plant may not return to its former glory. You can see how it looks after pruning and decide if you want to replace with another plant.