A few weeks ago I downloaded a file called Spanish Piece.wmv from a trusted source. It was a guitar instruction video. I have since deleted it. However, every time I start Firefox on my Windows 7 64-bit desktop, a zero-byte file named Spanish Piece.wmv gets written to my desktop. It will create a new one every time, and sometimes I have to go and delete a dozen or so of these files. Is there a batch file or startup file of some kind that this might have been written into? What do you suggest that I do to determine the cause of this problem? I would rather understand why it is happening rather than just make it go away by reinstalling and deleting all of my settings.
Try to delete the sessionstore.js file and possible sessionstore-##.js files with a number and sessionstore.bak in the Firefox Profile Folder.
Deleting sessionstore.js will cause App Tabs and Tab Groups and open and closed (undo) tabs to get lost, so you will have to create them again (make a note or bookmark them).
You can also try to delete the downloads.sqlite file in the profile folder.
Create a new profile as a test to check if your current profile is causing the problems.
See "Creating a profile":
If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.
I tried all those things, but now when I start up Firefox it writes five 0-byte files to my desktop, all bearing the names of old downloads.
Problem solved. Another user had a similar issue. Disabling the Add-On "Download YouTube Videos" made the problem disappear.