A few days back, Windows crashed while I was downloading a few files with Firefox and later on when I restarted my laptop, I was unable to download anything using Firefox. And that’s quite a pain because I love the bowser so much, especially with it’s host of useful extensions that aid with web development. So I looked around a bit and and managed to find the answer at Mozillazine which says:
If files downloaded don’t appear in the Download Manager, close Firefox, delete downloads.rdf in your profile folder, and try again.
It worked for me. Hope it works for you too if you ever require it.