What is the ruling on working in civil defense (firefighters) in a country that does not rule by Allah's law, given the obligation to swear an oath to uphold its system?

There is no harm for a Muslim to work as an employee for a state that does not rule by what Allah has revealed, or for a non-Muslim, if the work is permissible Islamically. Ali ibn Abi Talib, may Allah be pleased with him, worked as a hired laborer for a Jew. Similarly, there is no objection to working in the fire department (civil defense) because it is an honorable job intended to preserve lives and properties and to aid the distressed, and a Muslim will be rewarded for it if he purifies his intention for Allah. There is also no harm in taking an oath to uphold the system, meaning the civil work system that does not contradict Islamic law. It is permissible to make exceptions or impose restrictions that do not contradict Islamic law, if possible.