Is there a function like document.getElementById(\"FirstDiv\").clear()?
You should probably use a JavaScript library to do things like this.
For example, MochiKit has a function removeElement, and jQuery has remove.
You should be able to use the .RemoveNode method of the node or the .RemoveChild method of the parent node.
child.remove()For your use case:
document.getElementById("FirstDiv").remove()
This is recommended by W3C since late 2015, and is vanilla JS. All major browsers support it.
Mozilla Docs
Supported Browsers - 96% May 2020