Screenshot of the error and what site is this?

Hello,

Firefox won't download files from any websites. I need help to troubleshoot. Thank you.

Hey Mr. Jim L,

   Hope your day is going fine... Sorry to hear that you are unable to download files from any websites, and sure I'll help you troubleshoot this problem.

   1) First reboot and restart your computer as this process might refresh hard drive or ROM memory 
      with the desired memory address

    2) If still it doesn't work, then check the storage (in MB) in the folder you tried to save it, because 
        perhaps the storage is full, and try to store it in a different directory/folder. (It might also be due
        to that particular file extension that is not allowed in that particular OS [for example, .ad is not
        allowed in Windows OS])

    3) Check if you have any antivirus software in your computer, as it might interfere with downloads 
        and check if you have download permission from that particular website you're trying to download

    4) If the above three steps failed for you, try refreshing and restarting Firefox/try an updated version 
        of Firefox/try deleting any extensions possibly preventing Firefox from functioning normally

    Hope these steps might have helped you...

Let me know if you need more help if you still experience this issue, Tharakram.