For the first time, more people are expected to visit retailers’ websites through their smartphones than on desktop computers or tablets during the first weekend of the holiday shopping season, which began Thursday. The bumps in traffic and sales come as retailers try to make mobile shopping easier by improving their apps and adding coupons and other deals. Shoppers also have gotten more comfortable browsing retailers’ websites as smartphones have gotten bigger, making it easier for them to see photos of the items they want to buy. The National Retail Federation, a trade group for store owners, expects industrywide sales to be up 3.7 percent in November and December, less than the 4.1 percent of last year’s holiday season. Adobe, which measures 80 percent of online sales from the top 100 U.S.