I was fired from my very first job, when I was 16, because the boss changed my schedule, and forgot to call me about it. When I didn't show up for a shift I didn't know I was supposed to cover, I got canned. No big loss.
Years later, I was "fired" from a corporate job in a department that was continually being downsized. It was clear in my exit interview that all the bosses above me were looking for a cause for firing, but everything they threw at me, I could easily dispute, with proof. It was pretty amusing, once I realized they didn't really have grounds to fire me, and were just looking for an excuse not to have to lay me off, and pay me a severance package. When I filed for unemployment, the company disputed it, saying I was fired, so I took it to arbitration, and the labor board found in my favor: I was laid off, not fired for cause.
While I was waiting for the unemployment situation to resolve itself, I got a contract with a local publishing company to put together a book. When I turned it in a month later, they hired me full time, and matched my corporate salary. I used the firing/lay off scare as a heads up to get my finances in order, and spent two years at that publishing company, paying off all my bills. When I left to start my own company, I was debt free, and had a good six months of cash in the bank, in case I fell on my face. That was 11 years ago. I've been boss free, and hassle free, ever since. Losing that corporate gig turned out to be the best thing that ever happened to me.