Please solve the programming task below using a self-contained code snippet in a markdown code block.

You are given two lists of closed intervals, firstList and secondList, where firstList[i] = [starti, endi] and secondList[j] = [startj, endj]. Each list of intervals is pairwise disjoint and in sorted order.
Return the intersection of these two interval lists.
A closed interval [a, b] (with a <= b) denotes the set of real numbers x with a <= x <= b.
The intersection of two closed intervals is a set of real numbers that are either empty or represented as a closed interval. For example, the intersection of [1, 3] and [2, 4] is [2, 3].
 
Please complete the following python code precisely:
```python
class Solution:
    def intervalIntersection(self, firstList: List[List[int]], secondList: List[List[int]]) -> List[List[int]]:
```