As tomatoes grow taller, the lower leaves receive less and less light.  This sometimes causes these lower leaves to curl and brown. The best thing for your tomato plant, and for their appearance, is to trim off these lower leaves. This frees the plant up to focus on the top growth (where most fruit is formed) and keeps your plants looking healthy and vibrant. Frequent "rinse and refill" water changes can also delay the browning of lower leaves (see tips on "rinse and refill" for more details).