I am using Ubuntu 22.04. I have the default snap Firefox installed. When I download the external file (net.downloadhelper.coapp-1.6.3-1_amd64.deb) and do the installation, Video DownloadHelper does not recognize that the assisting file has been installed. After several hours of searching and reading, I finally found the solution from https://github.com/aclap-dev/vdhcoapp. I went to the following section:

Ubuntu Firefox: missed prompt

On Ubuntu, Firefox should have showed you a prompt to allow the coapp to communicate with the extension. Some people miss the prompt, or the prompt just never shows up.

There’s no user interface to change that setting. To solve this, run these commands (even if you don’t use Flatpak):

sudo apt-get install -y flatpak

flatpak permission-set webextensions net.downloadhelper.coapp snap.firefox yes

Ran: sudo apt-get install -y flatpak
The output:
Reading package lists… Done
Building dependency tree… Done
Reading state information… Done
flatpak is already the newest version (1.12.7-1).
flatpak set to manually installed.

Ran: flatpak permission-set webextensions net.downloadhelper.coapp snap.firefox yes
The output: Returns to the prompt.

I did not close Firefox or restart Firefox. I clicked on the Video DownloadHelper, clicked on the gear and then in the General tab clicked on the Recheck button. The page refreshed with the following results:
Found companion app: VdhCoApp 1.6.3
Companion app binary: /opt/net.downloadhelper.coapp/bin/net.downloadhelper.coapp-linux-64

Before I found the solution, these were the steps I took

Once the Video DownloadHelper plugin was installed, I tried to download a video. Video DownloadHelper prompted that a companion was required to continue with the download:

I downloaded the .deb package:

Once downloaded, I double clicked on the deb package and went through the installation. Once the installation was completed, I clicked on the Video DownloadHelper icon and clicked on the gear to get into the settings/configuration:

The following was shoing in the Companion App not installed: “Checking companion app returned: An unexpected error occurred.”

I then ran the following:
sudo apt-get install -y flatpak
flatpak permission-set webextensions net.downloadhelper.coapp snap.firefox yes

I did not restart Firefox, just went to the configuration of Video DownloadHelper and clicked on the “Reload” button, when the page refreshed:

Update 01/02/2024
I followed the same installation as above on a different computer and the install was unsuccessful. I did the follwing and the installation was successful. These instructons are on the DownloadHelper website: https://www.downloadhelper.net/install-coapp-v2

Since I am the only user on my computer, I used the “tarbz2 user-wide install.”

Linux x86/64bits tarball package

vdhcoapp-2.0.10-linux-x86_64.tar.bz2
Linux tarbz2 installation
tarbz2 user-wide install
# tar xf vdhcoapp-2.0.10-linux-x86_64.tar.bz2 -C ~
# ~/vdhcoapp-2.0.10/vdhcoapp install

Update 02/03/2024
Updated to vdhcoapp-2.0.10-linux-x86_64.deb and the plugin was not displaying as being installed.
I followed the instructions at : https://github.com/aclap-dev/video-downloadhelper/wiki/CoApp-not-recognized for Linux and followed the instruction:
Run /opt/vdhcoapp/vdhcoapp install (not as root! Don’t use sudo) to re-register the app.

$ /opt/vdhcoapp/vdhcoapp install
Installing…
Flatpak is installed. Making the coapp available from browser sandboxes:
Linked coapp within org.mozilla.firefox.
Linked coapp within com.brave.Browser.
Linked coapp within com.google.Chrome.
Linked coapp within com.google.ChromeDev.
Linked coapp within org.chromium.Chromium.
Linked coapp within com.github.Eloston.UngoogledChromium.
Linked coapp within com.microsoft.Edge.
Linked coapp within com.microsoft.EdgeDev.
Writing /home/blogger/.mozilla/native-messaging-hosts/net.downloadhelper.coapp.json
Writing /home/blogger/.var/app/org.mozilla.firefox/.mozilla/native-messaging-hosts/net.downloadhelper.coapp.json
VdhCoApp : VdhCoApp is ready to be used

The instructions from the site:

Linux (all)

• Run /opt/vdhcoapp/vdhcoapp install (not as root! Don’t use sudo) to re-register the app.
• Move to the installation folder of the coapp in your terminal, and run: /opt/vdhcoapp/vdhcoapp –info. It will run some diagnostic operations.
• do not install vdhcoapp within the /usr/ directory, it won’t be detected by your browser. See here why.

Update 05/27/2024
Another update!

Ran the following, as suggested by the app:
$ curl -sSLf https://github.com/aclap-dev/vdhcoapp/releases/latest/download/install.sh | bash
Downloading: https://github.com/aclap-dev/vdhcoapp/releases/latest/download/vdhcoapp-linux-x86_64.tar.bz2

Results:
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 –:–:– –:–:– –:–:– 0
0 0 0 0 0 0 0 0 –:–:– –:–:– –:–:– 0
100 52.8M 100 52.8M 0 0 41.1M 0 0:00:01 0:00:01 –:–:– 66.8M
Extracting tarball…
/tmp/vdhcoapp-ZBXbtG: bzip2 compressed data, block size = 900k
Registering CoApp
Installing…
Flatpak is installed. Making the coapp available from browser sandboxes:
Linked coapp within org.mozilla.firefox.
Linked coapp within com.brave.Browser.
Linked coapp within com.google.Chrome.
Linked coapp within com.google.ChromeDev.
Linked coapp within org.chromium.Chromium.
Linked coapp within com.github.Eloston.UngoogledChromium.
Linked coapp within com.microsoft.Edge.
Linked coapp within com.microsoft.EdgeDev.
Writing /home/blogger/.mozilla/native-messaging-hosts/net.downloadhelper.coapp.json
Writing /home/blogger/.var/app/org.mozilla.firefox/.mozilla/native-messaging-hosts/net.downloadhelper.coapp.json
Writing /home/blogger/.config/google-chrome/NativeMessagingHosts/net.downloadhelper.coapp.json
VdhCoApp : VdhCoApp is ready to be used
CoApp successfuly installed under ‘~/.local/share/vdhcoapp’.
To uninstall, run ‘~/.local/share/vdhcoapp/vdhcoapp uninstall’ and remove ‘~/.local/share/vdhcoapp’.
Re-run that script to update the coapp.