What was a major cause of the french revolution

There isn't one biggest cause for the French revolution, however major causes can be:

1. Political: French monarchy at the time considered themselves as 'representatives of gods' and had complete control over France. All kings before the French revolution (Louis XIV, XV and XVI) like unlimited power and arrested anyone they wanted to arrest. People were against that.

2. Social: France was a very miserable country during and before the French revolution, with society divided in three classes, the nobles, clerks and common people. Nobles and clerks had total authority and were the wealthiest. common people had to live in very bad conditions for their whole lives without anything to protect them from exploitation and poverty.

3. Economic: Due to past wars, France was in a huge debt. To reduce the increase in debt, France removed ministers, which only caused it to increase. As new ministers were appointed, the condition just worsened, with the debt increasing to nearly 1 billion Franks. The taxes caused lots of people from most classes to rebel, so the country could return to its great power. 

to protest slavery i would think it would be a