If you want to view the current HTML source in Firefox you can use Ctrl+A and then right-click > "View Selection Source".

I think it's more a question of what do you expect to see when you "View Source". For example, I have messed around with document.write a lot and it's pretty obvious to me that, if I use view source then it's going to give me the source and any changes done to it my document.write/open/close. In this case since document.write is used after HTML parsing has been completed it replaces the whole page and thus makes view source rather pointless.