AGB_FIRM seems to be like DS cartridge games in that it uses a different save protocol. While DS(i) cart support is simple, merely different, AGB_FIRM is notorious for stumping save managers. (Including Nintendo's own devkit one) - so just use the manual method. It's actually easier than dumping and injecting, which is the only way right now to decrypt GBA saves. (and when I say "dumping" I mean the NAND copy of the save, not the SD one)