I want to build an app. Should I just go with Firebase for the backend OR care making my own using Python/NodeJS etc...?

It won't be a small project IMO. That is, I should be able to get some users and hence, stay motivated to work on it, and keep adding features.

Will I miss out if I just go ahead with Firebase or Parse? I want to save time if possible.

SAVE TIME UNLESS YOU KNOW YOU CAN SHIP YOUR OWN SOLUTION VERY QUICKLY (not more than 2x than using firebase etc..)! DON'T BE ME. caps lock is a choice, I am screaming to you and to myself!

Thanks for your answer. It helps me realize that I should be able to ship my own backend quite quickly. I don't have prior Firebase experience, and though it was made to save time, that time saved would be ineffective in my case since I would have to do a lot of googling.

I assumed you knew firebase, but if you don't so goes with tools you are familiar. When your product will make money, you will be motivated enough in doing a transition if you need it