Understand and developing firmware - IDE help
As far as I know, Marlin - assuming that is the firmware you are using - is a standard C++ (the big brother of C) project that can be opened and compiled in the Arduino IDE, but also - at least - edited lots of other IDEs. 

Depending on your OS, there are several alternatives. Visual Studio, Atmel Studio, Qt Creator, XCode, Eclipse, Visual Studio Code, Atom, Sublime Text and the Arduino IDE are just some of them.

I also believe that this question should be moved to StackOverflow. However, if you have questions as to how you can make specific modification to Marlin, this forum is the right place to ask. If that is the case, consider reformulating your question!