Since I installed version 3.6.13, I can't download ANYTHING. Firefox shows the Downloads window and that the file is downloaded, but there is NOTHING, and I'm sure it's not an AV problem...
Do you have that problem when running in the Firefox SafeMode?
Don't select anything right now, just use "Continue in SafeMode."
If not, see this:
Make sure the download target under Tools, Options, General is valid.
Switch it to "Always ask me where to save files" as a test.
(happened to me once when I pointed it to a network drive and the network drive was no longer available..)