Is the Catholic canonized Bible closed? Traditionally most Christians hold that the Biblical Canon has been closed because of the end of the Apostolic era. But if the Pope speaks from "Ex Cathedra", isn't that stating his teaching is similar/equal to the Apostolic authority keeping the Canon open?
Correction


  The idea of a complete and clear-cut canon of the New Testament
  existing from the beginning, that is from Apostolic times, has no
  foundation in history. - Cf. Canon of the New Testament | New
  Advent.


A Primer on the Church's Infallibility

1) The Church's competency is faith [what we believe] and morals [how we are to act/behave in accordance with what we believe].

2) [Cf. Mt 28:20] All the Church was commanded to teach as to faith and morals by Jesus Christ is what is termed as the divinely revealed Sacred 'deposit of faith' (the depositum fidei) [= Holy Tradition + Sacred Scripture].


  [This] Divine  Revelation was given in its entirety to Our Lord and
  His Apostles. After the death of the last of the twelve it could
  receive no increment. - Cf. Revelation | New Advent.


3)  [Cf. CCC 2035] Christ's charism of infallibility to his Church extends as far as does the deposit of divine Revelation; it also extends to all those elements of doctrine, including morals, without which the saving truths of the faith cannot be preserved, explained, or observed. What this simply means is that the Church cannot err in what she teaches as to faith or morals, for she is our infallible guide in both. [Cf. Penny Catechism 100].

4) [Cf. Infallibility | New Advent] The organs of infallibility in the Church are:

a) the bishops dispersed throughout the world in union with the Holy See;

b) ecumenical councils under the headship of the pope; and

c) the pope himself separately.

Some facts:

1) In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced and defined that the Blessed Virgin Mary "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin." This was before:

2) The Vatican Council [1869-1870] which defined as "a divinely revealed dogma" that "the Roman Pontiff, when he speaks ex cathedra — that is, when in the exercise of his office as pastor and teacher of all Christians he defines, by virtue of his supreme Apostolic authority, a doctrine of faith or morals to be held by the whole Church.

3) By promulgating the Apostolic Constitution Munificentissimus Deus, 1 November, 1950, Pope Pius XII declared infallibly that the Assumption of the Blessed Virgin Mary was a dogma of the Catholic Faith.

Plain conclusions readily follow from above.

Clearing confusion in the question and answering it

When the Pope speaks ex cathedra according to the dogma of Vatican Council I, he can only do so only on those things that are already in the handed down deposit of divine Revelation which the Church in his generation has received through the ages from the Apostles or on all those elements of doctrine, including morals, without which the saving truths of the faith cannot be preserved, explained, or observed.

Councils have already determined the Sacred Scripture part of the 'deposit of faith' (the depositum fidei), no future organs of infallibility in the Church, pope included, can undo this.



Endnote

Holy Scripture as the Church has held it and how the Church inteprets it is a matter of faith:


  I also accept the Holy Scripture according to that sense which holy
  mother the Church hath held, and doth hold, and to whom it belongeth
  to judge the true sense and interpretations of the Scriptures. Neither
  will I ever take and interpret them otherwise than according to the
  unanimous consent of the Fathers. - Professio fidei Tridentina,
  Creed of Pope Pius IV [from the link, one of the four authoritative Creeds of the Catholic Church].