Where can I find old (historical) FIR map?
The best I can find is a ATC radar screen used in a dated report (appears to be a report by the Hong Kong Civil Aviation Department in 2000). It shows the FIR regions and waypoints of near Hong Kong, Hainan island and Vietnam:



Here is a dated enroute chart of Hong Kong in 1996. I'm not sure if the waypoint names will be useful for your further search:




How I end up with these: for the first one I used keyword "Sanya AOR". Given the dates you stated in the question, Kai Tak airport was still operational during that time. So I searched for charts of Kai Tak. A full set of charts of the airport would contain an "enroute chart" which lists waypoints. Search engine is Bing.