I dunno if this was actually getting fired, but this is the closest I've come to that:
The last band I played in had this "Russian Dragon" drummer, and to top it all off... he was a racist! I brought up my concerns regarding the drummer, to the band leaders, telling them I was uncomfortable around this guy, and he was an awful drummer, to boot. I couldn't believe it, but their response was, "well.. it's hard to find a drummer."
Anyway, I was going on vacation at the time, and I told them I'd think about where I stood with the band, and let them know when I got back (we had a gig about a week and half after I got back).
The guitarist calls me up one day before I get back from vacation, on my cell, and says, "Mike, we haven't heard from you, and we're concerned because we have a upcoming gig. We've been jamming with this kid..." It wasn't my intention to flake on the gig, I was committed to it regardless of the racist a-hole drummer, but my intentions were to quit the band after the show. When he told me they'd been jamming with someone else, I just laughed and told them to go with the other guy.
That was the easiest break-up of my life!