Go to the Mozilla Add-ons Web Page {web Link} (There’s a lot of good stuff here) and search for a download manager.

This download manager works separately from the browser. The browser tells it what to download, and off it goes.

http://www.freedownloadmanager.org It is a powerful modern download accelerator and organizer for Windows and Mac.

What security software do you have?

It is possible that (anti-virus) security software is causing the problem. Try to disable security software temporarily to see if that makes a difference.

Hi, I'm using a iMac and I don't have an anti-virus. For sure it is something else rather strange that is responsible for the problem.

Can you provide steps to replicate your issue?

Firefox is not able to download anything. It allways send a message saying that the download failed.

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

Starting Any Computer In Safe Mode (web link) Free Online Encyclopedia

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/ +++++++++++++++++++++++++++++++++++++++++++++++ If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

I've restarted in Troubleshoot Mode and the problem reported is the same. In Safari there is no problems with downloads.

Firefox and the computer?

Only Firefox has the download problem.

Try logging into another account on your mac and see if you have the same issues.