Two thoughts:
1) This is a self fulfilling prophecy. It's no secret that when it comes time to let people go (or time to promote), managers (even the MBA boogeymen) will stick their neck out for people they like more than for people they do not know. If you're continuously being dropped like a hot rock by your employers and you don't think it's because of performance reasons, perhaps you should reconsider your work relationship habits.
2) What's with the notion that you "lose your support network" just because you get fired? There is no reason that you should stop being friends with people just because you lose your job, even if those friendships stemmed from your workplace. I understand that it may be more difficult to find the time to socialize with them if you are not at work with them (which is exactly why it's worth it to go to those "forced beer nights" and build a relationship outside the context of work), but that doesn't mean you should stop putting in the effort and cut ties entirely.
Even if you are entirely career focused, getting fired is a situation in which you should actually lean heavier on your support network, especially coworker friends, as they are the ones that most likely can do the most to help you find a replacement job.