Do you get no more fire damage or only less fire damage in Minecraft with fire protection?

And can I swim in lava with it.

You can even swim in lava. I played a lot of minecraft back then.

There's complete fire protection. You can bathe in lava, set yourself alight and even fight fire without armor without any problems. You only have to be safe when the effects stop

You can also go swimming in lava

The enchantment gives you a little protection against it.

Fire and lava are still very dangerous.

Personally, I've never noticed a (big) difference…