Jennifer Rubin explains in the Washington Post why it might not be a good idea, from Trump's point of view, to fire Sessions:

"Not only would Sessions’s firing likely provoke an uprising on the right but it also would free up Sessions to cooperate with the special counsel and, like former FBI director James B. Comey, feel unburdened by executive privilege. Given that Sessions may have some liability of his own (giving false statements to Congress in confirmation testimony, omitting Russian contacts on his security clearance form, participating in a scheme to fire Comey under false pretenses, etc.), he would have every incentive to be as forthcoming as possible with former Senate colleagues now investigating the administration and with the special counsel."