Problem installing .deb in Software Install Ubuntu 20.04

After being downloaded, it goes into /tmp/mozilla_userX, from which the Ubuntu Software Installer is not able to install anything.

Just moving the file somewhere in /home/user/, e.g. Downloads, should solve the problem.

If you're looking for a reliable GUI way to install .deb files, use gdebi.

To see if you already have it installed on your system...

dpkg -l gdebi | grep ii

If it's not there, install it this way...

sudo apt-get update
sudo apt-get install gdebi

Then get the "Properties" of any .deb file, and set the default "Open With" application as gdebi. Now you can just double-click on .deb files to install them.

Note: gdebi may not operate properly if .deb files are on the desktop.

I'm experiencing the same problem, but if in the browser I choose "Save file" rather than "Open with: Software Install", then right click the saved file and open with Software Install, it works (it doesn't work with Ubuntu Software directly in either case).

Afaict, Software Install is part of Ubuntu Software