From what I have talked to people that write firmware: They all used C.
A classic example that I've loved to death: The fantastic FOSS mp3 player firnware Rockbox is mostly written in C.
I guess mainly for simplicity (little overhead) and speed.
Here's a relevant Quora link: Why is C preferred over C++ in firmware development
A short quote from a hackernoon article :
"Since C language was originally designed for system level programming, it’s no surprise that it’s commonly used to build operating systems and firmware.
It’s close enough to the hardware layer to manipulate low-level code but also fast and energy-efficient."