Firmware engineer questions:
1) Linked lists: reverse, find loop, insert (yes, NOBODY uses them)
2) Know ISR vs regular functions, what happens in a uP for each
3) Memory: Heap, static, auto, etc
4) Optimization of code: Loops, inlining, conditionals, switches vs if-then-else
5) Know how to write recursive functions vs loop-based, memory considerations
6) Boundary conditions in code
and since YOU ARE an ENGINEER, expect to be able to answer questions about the real world: Ohm's Law, Fourier analysis, integrals and differential calculus, averaging methods, probability, matrix math, control theory.