Is it permissible to pray for a longer life in order to witness the descent of Jesus (peace be upon him) and the major signs of the Hour, for the purpose of strengthening one's faith?

There is no harm in supplicating for a long life in obedience to Allah, but it is not recommended to supplicate for a long life specifically to see the Messiah, Jesus (peace be upon him), because this is linked to witnessing a time of tribulations, especially the great tribulation of the Dajjal. Instead, a Muslim should ask Allah for steadfastness in faith, frequently recite the supplication, "O Turner of hearts, make my heart firm upon Your religion," and be diligent in acts of obedience and righteous deeds to strengthen their faith.