Probably due to the horrible way animations are set up with setTimeout:

    var step = -1;

    function nextval() {
        step++;
        return 3000 + (20*step);
    }
    setTimeout(function() {

        graph.removeLink("31", "31_1");
        keepNodesOnTop();

    }, nextval());

There are 9687 setTimeout on that page.

almost crashed my Firefox 39.0 on Windows 7, very close.

i propose a new practice of putting (DJS) at the end of a link which might cause problem, like we do now with PDF file links. D stands for dangerous or danger or something. half joking here.

Doesn't happen anything after clicking the Start button in Firefox on Linux.

Crashes Pale Moon (25.2.0) on Win 7.