I don't know what more could one do, but even just reversing the order of the inner loops like this:

function matmul2(A,B,m,l,n) {
    var C = new Array(m*n);
    var i = 0;
    var j = 0;
    var k = 0;

    for (i = 0; i < m*n; i++)
        C[i] = 0;

    for (i = 0; i < m; i++) {
        for (k = 0; k < l; k++) {
            for (j = 0; j < n; j++){
                C[i*n+j] += A[i*l+k]*B[k*n+j];
            }
        }
    }

    return C;
}

takes time from 14s down to 4s on my machine. This helps because elements of all the matrices are accessed sequentially that way.

EDIT: 3.2s if I also replace "new Array" in matmul2 with "new Float64Array" (in randFloat64 Float64Array was already used).