It seems to me that a fairly large part of the fans disliked or detested Mourinho from the very beginning, back since his days in Chelsea. For being the enemy in PL, for his type of football, for the oil money poured into Chelsea and possibly some additional sins. Loyalists of course backed him as new manager in United. But he started with a minimum of slack. Slow progress and/or poor play made him lost the little support he had.
I think it is difficult to analyze the situation with JM and United. United was, and still is, in a much worse shape than most supporters believe. Players are seen as potentially high performers merely because they are part of the squad in United, albeit perhaps 40 % of them are better suited for a mid table team. There are no structures and procedures in the club, everything was tied Ferguson as a person, and chaos followed when he left. There is simply nothing left, apart from memories of a great past. A big difference compared to continental clubs like Bayern M., Juve or Barcelona.
There doesn't seem to be a plan for solving that "structural deficit" in the club. Managers are hired and fired in the waiting for success following big spending. I doubt that will come. Perhaps JM should be made director of fotball instead of manager? But I also find it difficult to understand JM in the present. No, he wasn't as cautious as he is now when he was Chelsea manager. In Porto, Chelsea and Inter he created a strong defense, a well balanced midfield and sharp counter attack. He has moved some way toward that, but the progress have been rather modest since he arrived. The recruitments (his decisions?) are not what you would expect from JM looking back. "Big names" but very unclear what they should contribute with in the building of a new United. The team play football in a halfbaked way. It is not old style Mourinho-football, rather some kind of neurotic and overly cautios caricature of that. He doesn't seem to trust his players and overbalance the team defensively. But he doesn't recruit better defenders or midfielders to make the team balanced.
Most supporters would probably cheer if he was fired today, or after the season, or anytime. But I fear that they will be equally disappointed by his successor. Perhaps it will feel better to be relieved from someone as disliked as JM was in the first place? The pains to come are perhaps easier to live through under manager X?