1) Don’t install anything other than via official channels/Ubuntu store or whatever else you trust, like Steam. More importantly, don’t add random PPAs from some guy on the Internet.

2) Boycott Firefox until they get their sandboxing act together. Use Chromium/Webkit2 browsers. Disable Java applets/WS.

Alternatively, go full Stallman[1].

[1] http://stallman.org/stallman-computing.html