Trump probably has been advised not to fire Sessions. So, he Tweets disapproval.
Sessions gave up his congressional seat to become AG. And the Trump administration forbids lobbying work for minimum 5 years after leaving the administration.
What we have here is a POTUS who has now got an embittered AG on his hands.
Congress told Trump there will be no hearings on a new AG in 2017 under any circumstances.
Trump does seem to provoke "Mexican standoffs", like the one with, well, Mexico!