Is what the questioner did—in informing the administration about his manager’s violations, and consequently being fired from his job—considered haram, a disruption of his livelihood, and fitna?

The primary course of action should have been to first advise the employee to change their behavior and rectify their mistakes. If they insist on their errors, especially if their conduct causes harm to others, then there is no objection to reporting them. The betrayal of a dishonest employee should not be overlooked; rather, they must be advised. Otherwise, their matter should be elevated to someone with authority over them to prevent corruption, negligence, and injustice, especially if their negligence leads to the disruption of work and the wrongful consumption of others' wealth. This falls under the category of offering sincere advice, changing objectionable conduct, and removing injustice. The concealment that is recommended is for those of good standing and those not known for causing harm or corruption. As for those known for such actions, it is preferable not to conceal their deeds; rather, their case should be brought before the authority, unless there is a fear of greater harm resulting from that. This is because concealing their actions emboldens them to cause harm and corruption.