Any browser that can be locked up by a bit of JavaScript that hogs the thread really needs to have NoScript enabled. The reason is there are too many easy ways to write a processor hog, either intentionally --

    function hog_processor() {
      while(true) {
      }
    } 

-- or unintentionally, by not knowing something about programming:

    function beginner_error_run_forever() {
      for(var x = 0;x != 10;x += .1) {
        // code here
      }
    }

You're right, but Today's World really relies on JavaScript and browsers development could, well, weigh it a bit more. Fighting on speed records might look good on browser's CV but for me personally doesn't matter zit - I'd take a stabler Firefox over a speedier one any second (I say this after 2 crashes today, on 2 different machines).