First of all, why do you need to fire employees to run a startup ? I don't think this is a major need in the development of a startup in fact firing someone is dangerous since you lose experience and knowledge.

France as many countries in Europe have laws that protect employees against abuse from companies but you can still fire someone if there is a reason it is just regulated.

You can fire at will for the first 4 month, renewable once, so practically 8 month, for IT people. It's plenty of time to determine whether someone is fit for a job or not.

If you want to fire someone after those 8 month, you indeed have to justify your decision (economical, huge mistake from the employee, common accord) and follow the process, but it's doable as long as you actually have a justification

You can, you just need to do it through the performance management or redundancy processes and not on a whim. You can also usually have "probation" periods of six months or so.

(also "Europe" is a large place, could you be more specific?)