From a purely objective point-of-view - It will be counter-productive to fire Mourinho at this point. Simply because you do not have anyone that will be a such a remarkable upgrade on Mourinho that he can take up this current squad mid-season and perform wonders. As fans, you should back up the manager and the squad and hope for the best.
However, I do disagree with people defending Mourinho with 'It is not him, it is the players' or 'He wasn't backed in the transfer market'. I mean come on! There is no doubt that Mourinho is under-performing.
As a manager, it is his core job to get the best out of his players and improve them. If he is not able to do that, it is his fault. Also, regarding the spending, it can be an excuse for not being able to compete with City but it is not as if he has not spent or brought in players he wanted. Considering he has had the money and the time to revamp the squad, if United are still bogged down by mediocrity, then the fault lies, at least partly, with Mourinho.
FB - Dalot
CB - Baily, Lindelof
Midfiled - Pogba, Matic, Fred
Attack - Mkhi, Zlatan, Lukaku, Sanchez
These are his big money signings. All of them brought in for the first XI. (Except for Dalot maybe). Like it or not, the current squad is a Mourinho squad.