Cannot install .deb packages opened from Firefox

[Aren Cambre]

Suppose in Firefox you click a link to a .deb package. A dialog named "Opening [filename].deb" opens. If you select "Open with", then the .deb file is downloaded, placed in /tmp/mozilla_[username]0/ directory, then Software Install (as named in the Firefox dialog) is started on that .deb file.

Next, Software Install loads and fails with "Failed to install file: not supported".

If on the other hand, in Firefox's "Opening [filename].deb" dialog, if I select "Save File" instead, the .deb file ends up in the user's Downloads directory. Software Install works fine with the package there.

I don't want to get too much into solutioning or "why". The main point of this bug is that I should not get a weird error just because I selected "Open with" instead of "Save File". This happens with a vanilla install of Ubuntu, with nothing else installed, so the product isn't working in an intuitive way.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 5.8.0-33.36-generic 5.8.17
Uname: Linux 5.8.0-33-generic x86_64
ApportVersion: 2.20.11-0ubuntu50.3
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: ubuntu:GNOME
Date: Wed Dec 16 19:54:01 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
InstallationDate: Installed on 2020-12-10 (6 days ago)
InstallationMedia: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: ubiquity
Symptom: installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Aren Cambre (aren-arencambre)]

Some have reported this at https://askubuntu.com/questions/1232868/problem-installing-deb-in-software-install-ubuntu-20-04

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[PhoneixS (phoneixsegovia)]

In my case, it could be because "Ubuntu Software" is really "snap-store" and for some reason, it can't install it from that directory. Could be that the problem is on snap-store and not in Firefox?

[Sebastien Bacher (seb128)]

The snap store as a snap isn't going to have access to the share /tmp so that would be an issue yes

[thinkpad (fellowsgarden)]

tmp will not work for some applications; any link explaining exactly why/when/how? is it just because of snap - or other things too?

[Cas (calumlind)]

As others have suggested this is a snap app issue with /tmp dirs and can be verified with:

gnome-software --local-filename '/tmp/mozilla_user0/app_amd64.deb'

snap-store --local-filename '/tmp/mozilla_user0/app_amd64.deb'

I have added workaround to AU to use 'gnome-software' for opening debs: https://askubuntu.com/a/1330582/8570

I did also find an alterntive workaround so that all snap apps have access to /tmp: https://askubuntu.com/a/1264341/8570