Specifically set `.chat-body` height

Otherwise the private chatbox header doesn't show in Firefox

css/converse.css

@@ -9554,6 +9554,9 @@ body.reset {
   #conversejs.converse-overlayed .chatbox .box-flyout {
     min-width: 250px !important;
     width: 250px; }
+    #conversejs.converse-embedded .chatbox .box-flyout .chat-body,
+    #conversejs.converse-overlayed .chatbox .box-flyout .chat-body {
+      height: calc(100% - 55px); }
 
 #conversejs.converse-overlayed .flyout {
   bottom: 1em; }

sass/_chatbox.scss

@@ -495,12 +495,16 @@
     }
     
     .chatbox {
-        min-width: $overlayed-chat-width!important;
+        min-width: $overlayed-chat-width !important;
         width: $overlayed-chat-width;
 
         .box-flyout {
-            min-width: $overlayed-chat-width!important;
+            min-width: $overlayed-chat-width !important;
             width: $overlayed-chat-width;
+
+            .chat-body {
+                height: calc(100% - #{$overlayed-chat-head-height});
+            }
         }
     }
 }