1) Storage space decreasing ?

I realize that my storage space is increasing.
I have disabled background data and it decreased from 60+mb to 50+mb.
However, whenever I go internet or use Youtube, the storage space will go up. After clearing history, cookies etc., it goes down again, but it doesn't go down to the amount that I see before I go internet or use Youtube. I guess this is normal ?

2) Upgrade firmware

I am wondering what will be lost when we upgrade the firmware of the phone ?

Thank you.