The reasons for high unemployment in France are, I would suggest, more to do with how hard it is to fire people in France than taxes. If you know you'll be stuck with someone forever, you take the decision to hire much more seriously and you'd much rather ask an existing employee who's a known quantity to do more work instead (potentially even if you pay them more in raises than it would have cost to hire someone else).

Contrast that to e.g. Denmark where taxes are arguably higher but making redundancies/reducing workforce is not anything close to as hard or expensive for the employer.