@limotux let us know how it goes with the Firejail GUI.
@BluishHumility if the firejail profile prevents the use of downloads directory and other directories under $HOME then it hardens the browser more, because it might prevent the ability to download and upload files. Thus preventing the spread of malware via attachments. Maybe it means that Firefox profile is more permissive compared to Chrome based browsers or in other words, it hardens Chrome based browsers more than it does Firefox.