The first line in the .hex file must set the start address. The firmware file has this as line 1:
:020000023000CC
If you open up the downloaded file and add that as line 1, then disassemble it, it changes the start address:
Mega2560.hex: file format ihex
Disassembly of section .sec1:
0003e000 <.sec1>:
3e000: 0d 94 89 f1 jmp 0x3e312 ; 0x3e312
3e004: 0d 94 b2 f1 jmp 0x3e364 ; 0x3e364
3e008: 0d 94 b2 f1 jmp 0x3e364 ; 0x3e364
3e00c: 0d 94 b2 f1 jmp 0x3e364 ; 0x3e364
3e010: 0d 94 b2 f1 jmp 0x3e364 ; 0x3e364
...
3e0e4: 41 54 subi r20, 0x41 ; 65
3e0e6: 6d 65 ori r22, 0x5D ; 93
3e0e8: 67 61 ori r22, 0x17 ; 23
3e0ea: 32 35 cpi r19, 0x52 ; 82
3e0ec: 36 30 cpi r19, 0x06 ; 6
3e0ee: 00 41 sbci r16, 0x10 ; 16
3e0f0: 72 64 ori r23, 0x42 ; 66
3e0f2: 75 69 ori r23, 0x95 ; 149
3e0f4: 6e 6f ori r22, 0xFE ; 254
3e0f6: 20 65 ori r18, 0x50 ; 80
3e0f8: 78 70 andi r23, 0x08 ; 8
3e0fa: 6c 6f ori r22, 0xFC ; 252
3e0fc: 72 65 ori r23, 0x52 ; 82
3e0fe: 72 20 and r7, r2
3e100: 73 74 andi r23, 0x43 ; 67
3e102: 6b 35 cpi r22, 0x5B ; 91
3e104: 30 30 cpi r19, 0x00 ; 0