Summary: Here's everything you need to know before Thursday's opening bell: Walmart said it's raising its minimum age for firearm purchases to 21 and has removed both firearms and toys that look like assault rifles. Exxon Mobil XOM said it would exit some joint ventures with Russia's Rosneft. Spotify filed to go public. Best Buy plans to close all of its 250 mobile-phone stores in the U.S. U.S. stock futures suggested a rocky start following Wednesday's triple-digit decline. Watch TheStreet's investing interviews on Youtube
