@llanaro 

I invited myself as "accountant"

Using a totally different email address or using the same email address that you've as admin? If it's a new email address, you should be able to delete like the other one on the Accounting firm list. 

Since they recently updated this manage user, I hear issues, not sure they fixed it yet. I would suggest calling Tech Support and let them look at it. Hope they resolve this for you.