Former senator William “Mo” Cowan (D-Mass.) — who was appointed in January to fill the Senate seat of Secretary of State John F. Kerry when he moved to the State Department, and who left the Senate in late June after then-Rep., now-Sen. Ed Markey (D) won a special election — is headed back to the Boston-based law firm Mintz, Levin, Cohn, Ferris, Glovsky and Popeo to run their government relations and strategic consulting affiliate and be of counsel to the firm.