Have you ever been fired from IB?

not fired, but eventually many end up in a dead end position. sometimes a team doesn't have opportunities for advancement, others teams may not hire, and maybe you'll see a role you could go for externally.

What would someone have to do to get fired in an IB?

I think it's pretty hard to get fired at the junior level (analyst/associate). If as an associate it becomes clear you'll not be promoted to VP, people are slowly pushed out (no bonus, no responsibility given, etc.) and as an analyst, it's just not worth it. I think you would very rarely be "fired" 

At a more senior pre-MD level, things can be a bit more delicate. I would typically think that being a director is the hardest position, as you need to manage the day to day of many different accounts, all the while prove you can generate new leads. Many people don't make it past this final hurdle, and they are easily pushed out. Typically they go a very solid step down in terms of bank/group prestige and maybe make MD somewhere else. 

It's rare for analysts to get fired. The summer internship is supposed to act as a filter for candidates who don't gel well. If an analyst underperforms in their fulltime role, they get labeled bottom bucket and languish-probably get advised to leave gracefully. There are things you can do to get fired immediately though, doesn't happen very often-insider trading is one of them.

One exception is a financial crisis like '08, where some BBs fired analysts right after their training program (S&T though, not corp. fin though for what it's worth.)