No question that the Sessions firing was intended to impede or halt the investigation into Trump's affairs and the affairs of his family. There may be a much larger game afoot here from the Russians. Trump is heavily indebted to the Russian mob and Saudi royal family and he has always placed his welfare above that of others, even his son Don Junior. Trump has probably been a Russian asset since the early 2000's when he was on the verge of his biggest bankruptcy. The banks in North America and Europe would no longer touch him and he was desperate. Enter the Russian mob/big business/government. Eric Margolis, the noted reporter who has been watching the Russian and Asian political scene for some 30 years, maintains that the Russians have been far more successful in penetrating the American and British intelligence and political machinery than the CIA or MI6 ever were in accessing the Russian equivalents.
Russia has succeeded in influencing American foreign policy beyond their wildest dreams, but their real endgame may be to destroy American societal cohesion and set the different factions at each others' throats. Trumps denigration of his intelligence community and discounting of their advice is another step in undermining trust in these institutions while he looks the others way. The prime and recent example of the hither-to respected FBI being ordered "investigate" the allegations against Kavanaugh and then being manipulated so as to produce an exculpatory report on Kavanaugh with them interviewing Kavanaugh, Dr, Ford, Kavanaugh's best friend or either of the other two women who came forward, has to undermine Americans' trust of even the FBI. All this is likely intended to produce exactly what has happened- fatalism on the part of voters that no matter how or if they vote, it produce no positive change. Then, if there is no trust in democracy, all that is needed is for a semi-charismatic leader to step forward with a simplistic "plan" and extravagant promises and voila! we have Germany at the end of the Weimar Republic. America then either becomes a thinly disguised dictatorship or a helpless giant ripe for the picking.