Just to clarify and to be useful to others that may have the same issue.
Yes initially it was installed, but during the time it did not work and I tried various suggestions, I uninstalled and re-installed the AUR vdhcoapp-bin a few times.
The last time before I applied your suggestion, I wanted to start clean. so I uninstalled the vdhcoapp and I ‘refreshed’ Firefox. I then had to do all the customisation of Firefox settings as it was before. after that I re-installed the vdh extension, then installed the AUR vdhcoapp-bin. I know that Linux does not need to ‘re-start’ like Windows, but just for good measure I re-started after the install. After that the app was still not installed/working!? That’s when I run your command (/usr/bin/vdhcoapp install --user) and after that all was working.