I'm not sure whether this one counts as being fired or not.
In addition to my main job as a DJ/presenter and commercial producer, I used to have a small number of regular voiceover gigs on the side. One of them was narrating a weekly TV show that aired in the UK. The production team spent all week putting the show together and writing the script. I would visit once a week and, for an hour, sit in the voice booth, read the script to video, and leave.
I was never very good at voiceover. If my natural voice didn't exactly match what the client was looking for, I had limited ability to change my delivery in order to match what the client wanted. In this case, the client in London wanted a punchier delivery than I was capable of. I was always aware of this. Occasionally, they would encourage me to "amp up" my delivery a bit. I would try, but just didn't have it in me.
One day, after my regular weekly VO session, the director called me into her office and gently informed me that my services were no longer needed. She went out of her way to soften the blow by telling me that, were they to need me in the future, they'd be certain to call me back. As I walked out of her office, several of the producers offered their condolences, and told me they were so sorry. I assured them it was fine, and I didn't really mind, which was true.
It wasn't until some time later, that I realized I had been let go. It never felt like that though, because it wasn't my main gig, and I didn't rely on it for my livelihood - it was all extra income. Furthermore, a lot of voiceover jobs are one-off. You audition for a bunch of jobs and, if you get one, you do it, then it's back to searching for more work. It's like acting in that respect. I was tickled pink that I'd landed a regular, recurring voiceover gig. I was always aware that I wasn't really giving them what they wanted. Every week that I went in and got paid for reading a script into a microphone, I considered a bonus. I had just bought a house and incurred quite a lot of extra expenses. That 18 months - 2 years of extra work paid off all my debt at the time (apart from the mortgage). It was all upside.
It wasn't a bad thing when that gig ended. It was one less thing to think about - one less commitment. I guess, in a way, I was fired, but it didn't feel like it.