I had used a disassembler a long time ago, but it has apparently vanished from my computer, so here is the first page of Google search results where I found 8051 disassemblers, which is how I would go about obtaining a disassembler again if I needed one. If they don't accept Intel HEX format as input, they're likely to accept binary format. If you don't have a HEX-to-BIN conversion utility, those are also freely available.