No research indicates that briefs (or any other underwear) have an impact on fertility. The temperature in the scrotum is usually a bit cooler than normal body temperature. Higher temperatures can affect male fertility. Some research states that briefs cause a higher temperature in the scrotum but no research states that this in turn, affects fertility.
If you are trying for a baby, it is generally recommended that you avoid high-temperature places like saunas and hot tubs, as they can affect sperm production and fertility­.