After login in the HTML5 client you have three items in the Firefox browser history. Two which both lead to the login page on the master server and one which is the actual client on the agent. In Chrome, Safari and Internet Explorer only one item for the login page is created in the history, this is the desired behavior.
The two items seen in Firefox are likely caused by the two POST which happen during the login process.
Fixing this would require architectural changes. Maybe use WebSockets instead of the first POST?
This seems to work fine in current versions of Firefox (tested with 80.0), where I now only get a single history step from the login window to the session.
The redirection page is a bit oddly handled in the code though, so I'll use this bug to do a bit of a cleanup.
All done and redirection still works fine.
Redirection works fine, no duplicate entries in history either. Tested on Firefox 81.