https://www.zimbra.com/ distributes mail servers with a web client, as used e.g. by telenet.be , a daughter company of Liberty Global. The issue is that the body of HTML mails does not show (except for the first half line) in Firefox in the main window. The window has an option "show in separate window" where it works correctly. I have this issue every time when I reinstall Firefox. After some weeks the problem tends to disappear and the window shows correctly. At the same time Firefox recognises this as a mail client and proposes the site as an alternative when I use the "email a link to this page" button. I have this on different (Windows 10) PCs I use, for a couple of years. And I would like to get rid of it.