• Check the Downloads panel for blocked downloads. ...
• Clear the download history. ...
• Choose a different download folder. ...
• Reset the download folder. ...
• Change file type settings. ...
• Configure Internet security software. ...
• Other solutions.

https://support.mozilla.org/en-US/kb/cant-download-or-save-files

Dear Pallavi, thank you very much for your responses. I tried them all. This is not the problem. The point is that I can download even large files. But the downloads stall when I do not move the mouse. All your suggestions did not solve the problem.

Alfred

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.

You can do a malware scan just to be sure.

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

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