Your coapp install is OK and I think i understand the problem.
It appears that Canonical (the editor of Ubuntu) decided that since 21.10, the default Firefox package will be installed as a snap package. In this install mode, Firefox runs in its own sandbox, with no access to common directory in the filesystem. This means the browser simply does not have access to the coapp. This is bad news we haven't seen coming.
For now, the workaround should be simple: uninstall the default Firefox from your system and reinstall it from the .deb package. This should work:
sudo add-apt-repository ppa:mozillateam/ppa
sudo apt-get update
sudo apt-get install firefox
Another alternative, and it hurts me to say so, is to move from Firefox to Chrome.
We'll investigate to see how/if it is possible for the coapp to be usable in the Ubuntu snap context.
Thanks for reporting the issue.