I don’t know as I concur ______ your decision to fire the manager.

'Concur in' is followed by the decision or the agreement.
'Concur with' is followed by the person sharing a decision or agreement.
Ex:
1. We concurred in the decision to go ahead with the project.
2. I concurred with the project manager on the decision to go ahead with the project.

তেমনিভাবে,
I don’t know as I concur with your decision to fire the manager.