Re: Howard says that not getting to Reprise his Ironman role killed his career!
To be fair, Norton didn't get the axe. He signed for one film. They asked him back but he wanted more money than what was offered.
Considering how huge the film was, and what it could have done for his career, (vs. having a bit part with Renner in the last Bourne movie), it's surprising that he turned it down. But he didn't quit and he wasn't fired.
Howard on the other hand was contracted for 3 films, plus Avengers. Cheadle was also contracted to be in Avengers, but when the story didn't go that way, he was released from that obligation.
Howard was released from his contract (i.e. fired) in Megan Fox type fashion.
"I'm going to show you something beautiful. Everyone screaming, for mercy. You want to protect the world, but you don't want it to change. You're all puppets tangled in strings.......strings....... but now I'm free. There are no strings on me."
- Ultron V