In Europe, we don't really have the concept of at-will employment so an employer can't legally terminate your employment without a good reason. And resigning isn't usually classed as a good reason.

I'm not sure about Spain, but in the UK there is a difference between being fired i.e. terminated with cause and laid off i.e. made redundant.

In the former, I don't believe there's any requirement for severance. In the latter there is a statutory minimum severance.

Also, if you're actually fired, your rights to unemployment benefits change. Usually some form of delay and/or reduction.