How do I install Mozilla Firefox to my PC?

You can use Ubuntu's package manager to update the already installed Firefox. Open a terminal, and enter the following commands one by one.

sudo apt update
sudo apt install firefox -y

You can update the whole system with

sudo apt dist-upgrade

Alternatively, if you prefer GUI, use the Update Manager to update apps.

You cannot "install" the version of Firefox you download from the Mozilla website. You can run the executable file inside it, but it is much simpler to use the default version. If you still want to use this method, head over to the manual installation section in the Ubuntu Community Help Webpage.