Hi,
I'd assume that as well - and they probably ARE the same but I guess double checking it wouldn't hurt (I don't have any old version of the compiler on this machine so I can't check).
When you say that the it works with the .hex from 2.5c are you actually compiling the code with 2.5c and using the resulting .hex file or are you using a previosuly compiled and saved .hex? The reason I ask is because you might, at some point in time, have used MPLAB to change the CONFIG and saved it back into the original .hex.

Anyway, the datasheet for 876A says that you should use HS_OSC for 8MHz and up - my guess is that's where the problem is.
Import the original .hex file in MPLAB and look at the CONFIG (Configure->Configuration bits), does it say XT or HS? Then do the same with the new .hex, if it says XT change it HS before programming the chip. If that does the trick then take a look at the #CONFIG directive in the manual and use it to change the CONFIG in your code.

/Henrik.