In order to fix the bug in Firefox (https://bugzilla.mozilla.org/show_bug.cgi?id=1953102), add CSS statements to have the scrollbar

Author: mmoqui

core-war/src/main/webapp/util/styleSheets/silverpeas-main.css

@@ -7908,4 +7908,12 @@ input.dateToPick,
 .spRating.rateit {
   height: 16px;
   overflow: hidden;
-}
+}
+
+/*********** For ConverseJS as a workaround of a Firefox bug */
+@-moz-document url-prefix() {
+  .conversejs .chatbox .chat-content .chat-content__messages {
+    overflow-x: auto;
+    display: block;
+  }
+}