Linux

Cinnamon is a little heavier on the hardware but especially with Linux Mint it's a great choice to get into Linux and a Debian base is a great choice, you won't utelize any new features on that hardware and it's really stable! About KDE, that's actually very light weight too so you shouldn't have any issues in that regard if you decide to use it especially in comparison to Cinnamon, I would just recommend Mint Cinnamon more for a new user! :)

With regards to performance, the distro matters but the Desktop Environment matters more.

For older hardware, XFCE is going to perform a bit better than GNOME. KDE is a middle-ground. Choose what DE you want first and then go for a distro that runs that well.

I like puppy Linux. It can be a bit quirky but it is very very fast.

Here is an oddball solution, the lightest way to have GNU/Linux and be able to use GUI applications like Firefox is to simply start a bare bones os install X-Server and something like dmenu, That's it. Suckless.org, there is a lightweight dwm, a desktop window manager that you can use to tile windows and move them around and more. dmenu will be used to just launch the application. dwm is what manages the windows. Anything past that is based on what you need. It can be a fun challenge to make the most lightweight Firefox browser launcher.

For now, stick with what others have suggested. bare bones installations are usually meant for helping you single out a task and usually offer poor multitasking features until you put a lot of effort into installing and configuring more packages to a satisfying ease of use level.