Can I get a JSON value from the internet without flashing a custom FW on EV3?
The easiest way to do this is with a 3rd party operating system. On the EV3, these generally run from a microSD card and do not touch the firmware in the flash memory of the EV3, so technically, you are not flashing a custom firmware (think dual-boot). The major 3rd party OSes are ev3dev, leJOS and MonoBrick. All of these have Internet access from the brick itself.

To do this using the official firmware only, you won't be able to access the Internet directly from the brick. So, you will have to have an application on a computer or a smartphone that accesses the Internet and relays information to the EV3 using Bluetooth messaging.