Any ideas how to make this cardboard shelf firmer???

Get some foamboard dollar store or craft store. Cut panels to fit, tape or glue to sides.

Adding a strudier back would also help reinforce this.

Apply glue saturated cloth to the back, let it dry and BAM!!!

cut or drill small holes and use wooden dowels through the entire unit straight across Could paint the dowel a cool color to spice it up

https://www.youtube.com/watch?v=-OMd7NDahOE

To firm it up and glam it up at the same time... insert a dollar store frame into each opening.

LET THE OLD HORSE DIE IN PEACE-- use it for firestarter! Spend your hard earned money on wood, metal, or someting STURDY to begin with --- make stacking shelves out of wooden crates-- from JoAnn Etc .. get coupons daily off internet email-- for half price on anything -- and purchase at a discount! Save money by being a frugal shopper! AND- you'll never have to re-do the shelves again. You can leave the wood natural even, without any finish, OR paint, stain, or whatever. I built a giant book-shelf-unit out of crates. It looks stunning, is sturdy, and for safety purposes- lash crates together with plastic strips (hide them in the back, behind "stuff" -- and you are GOOD TO GO!!!!!

I'm not sure about making it sturdier, but you can keep any heavy items off the top of the shelf so it doesn't put pressure on the cardboard.

Hi Wen, you can make cardboard super hard with glue. This video will show you how - https://www.youtube.com/watch?v=UbPLNUrVfkU