But firmware? Totally different. FreeRTOS does have a POSIX backend, which helps some. Maybe you can run it under a hardware emulator. But it seems like lots of the stuff you want to test isn't really testable from the perspective of what a typical dev knows.
A lot of firmware dev iteration seems to be build -> flash -> watch serial connection for debug prints.
Serial port? Maybe for the fancy folks. Us lunch-pail types would find an unused GPIO, blue-wire an LED, and blink out a code of your choosing.
Okay, I never actually had to blink an LED but it was in my bag of tricks. The real golden-ticket to debugging embedded devices was a development kit with JTAG. JTAG, coupled with expensive additional hardware and equally expensive software license, gave you a gdb-like interface for debug. Breakpoints, stack traces, all the good stuff you take for granted when working with Windows, Linux, or those ugly abominations we call web browsers.
Emulators were also a thing, especially if your product had a custom ASIC and you needed pre-silicon development. But I didn't use them a lot myself and it seemed like by the time you had your emulation environment setup, there was 1st gen silicon and a debug board sitting at your desk.
Not to say it isn't a valid way to debug, but there are definately better options available.
Iv dabbled in some basic MIPS assembly and some microcontroller programming but don't consider that as complicated as boot level firmware or say the firmware that controls complex stuff.
I view embedded as the closest I will ever get to actual hardware engineering and it shocks me how complex everything is.