Firefox cannot connect to WSS

I have a websocket backend server with Spring Boot Websockets. Safari, Chrome and Edge can connect, but Firefox cannot. Mistake:

Firefox kann keine Verbindung zu dem unter server wss: // MY_DOMAIN / growth-websocket / 933 / omw002tp / websocket aufbauen.

(= "Firefox cannot connect to server in wss: // ...")

I am proxying WebSockets with Apache:

RewriteEngine on
RewriteCond %{HTTP:UPGRADE} ^WebSocket$ [NC]
RewriteCond %{HTTP:CONNECTION} ^Upgrade$ [NC]
RewriteRule .* ws://localhost:4567%{REQUEST_URI} [P]

      

        
        
        
      

    

Thank you for your help!