I don't believe so, but you can change Firefox to ask where to download stuff to, instead of the automatically downloading to the default download folder. It will instead being up the save as dialog box where you can enter a different filename.
Oh this is to save me the steps of "open containing folder" then press f2 to rename
So the save as dialog on every download wpuld be cumbersome for me.
I was mostly checking in case it already existed. This looks like a reason to create my first firefox addon !