Well, you can't really be fired that easily. If you want to leave anyway, you don't need to do that, but the recommendation is usually to go to a lawyer. 50% of all dismissals have formal errors, of the others, 50% can still get something out of the conciliation hearing, and 50% of the cases that actually go to court are won by the employee. Although winning usually just means a bigger severance pay. Normally, the relationship is then broken beyond repair.