[Xav]

Your code logic is wrong. What you're saying is this:
If the process is called star, then check if the process is called firefox as well. If so, kill the process.
You see the problem?

Consider using a switch statement instead of multiple Ifs.

switch (p.ProcessName.ToLower())
{
 case "star": {}
 case "firefox": {}
 case "other":
 {
  p.Kill;
  break;
 }
}