Snagging the top job at Twitter would be a triumphant return to the helm of the microblogging service that Dorsey helped invent but from which he was previously fired as CEO in 2008. News reports have said that Dorsey, who at times has expressed interest in careers from fashion designer to New York City mayor, struggled to manage frequent outages during a period of exploding growth at the service. Wall Street investors reacted positively, briefly sending Twitter’s shares up as much as 6.8 percent.