Is there a Firefox extension that add a rename feature (for example a textbox) to Firefox's
Download Dialog ?
If you right click on a link and select
Save Link As you will get the option to name the file as whatever you want.
Another option is the DownThemAll extension. This gives you much more control over your downloads including renaming.
You may not need an extension if all you are trying to do is just give a name to a file as you start to download it. In Firefox:
See Tools > Options > General|Main : Downloads : "Always ask me where to save every file"
Answer was taken from https://support.mozilla.com/en-US/questions/749403#answer-31655.