Quick A: They do not.
Recently on SO:
Known problem. C++ mutexes are thin layer on top of OS-provided mutexes, and OS-provided mutexes are often not fair. They do not care for FIFO.
The other side of the same coin is that threads are usually not pre-empted until they run out of their time slice. As a result, thread A in this scenario was likely to continue to be executed, and got the mutex right away because of that.