Surely open-source handset OSes are just as vulnerable to patent attacks? If Firefox OS were to take off, it would immediately become a target for exactly the same sort of lawsuit.

The question was not how do I buy a smartphone which is invulnerable to patent attacks/lawsuits, but how can I avoid supporting this kind of predatory corporate behaviour (Apple, Google, MS were listed as examples).

I agree open source handsets (as far as they can be) would also be vulnerable to software patent attacks, but we could at least support companies which don't engage in such attacks.