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.
“Whenever you see somebody crossing over to something it gives it pop. Like Murder She Wrote and Magnum [P.I.] — great crossover. I got excited.” Joss Whedon on Spider-man joining the Avengers.