I have a fresh install of Ubuntu 18.04.3 LTS and Libreoffice 18.104.22.168 (the latest in the repository). When I try to do File->Preview in Web Browser, nothing at all happens in Calc or Writer.
I have firefox and qutebrower and falkon installed and my $BROWSER environmental variable is set to qutebrower (I tried setting it to firefox and still nothing worked).
I have tried “$sudo update-alternatives --config x-www-browser”, selecting firefox, qutebrowser, and falkon. None of this worked either.
I only ever use calc for simple spreadsheets, so I don’t really have any experience with it in terms of its settings. I poked around in Tools->Options->Load/Save->HTML Compatability, but nothing there seemed to help.