That's a very good kind of random number generation, provided that there is some amount of user interaction so there are some irregular delays during which the timer continues to run. If you just start a timer when the 8051 starts and then the program runs at a speed independant of any external source, it's very likely that you'll get the same numbers each time.

One way to do it if there are no random delays introduced by the user would be to use some location in the internal RAM after a cold start to seed a random number generator - that data is believed to be pretty random after a cold start.