Apparently he was, after the recording off the Live Evil album.
Tony Iommi and Geezer Butler accused Ronnie James Dio and Vinnie Appice of going into the studio and turning up the volume level of their parts on the recording tape. That was their grounds for letting them both go.
I suspect they regretted that decision when the sales of their subsequent albums tanked! Firing Ronnie James Dio ended up costing Iommi and Butler a lot of money in lost income.
They came to their senses and hired Ronnie back for the Dehumanizer album, only to screw up royally again - this time due to a situation involving Ozzy Osbourne. They were actually going to have RJD sing for them in an opening slot for Ozzy for two shows and then to really make things worse, they were going to reunite with Ozzy - a blatantly disrespectful thing to do to Ronnie James.
Dio quit and went back to fronting his own band - Dio.
Iommi and Butler floundered even worse without Dio than they had the first time! Fortunately for them, Ronnie James forgave them and came back to make music with them again - this time under the name Heaven and Hell. He stayed with them until the end of his career.