If RadEditor is initially hidden with display:none, its tools do not work when displayed in Firefox 62.0:

<div style="display: none" id="hiddenWrapper">
    <telerik:RadEditor ID="RadEditor1"  runat="server"></telerik:RadEditor>
</div>
<script>
    function f() {
        $get('hiddenWrapper').style.display = "block";
      }
    Sys.Application.add_load(f);
</script>


There is a warning error in the console: "Mutation Events is deprecated and to use MutationObserver".

The fix is to call the $find("RadEditor1").onParentNodeChanged(); method to recreate the content area:

<div style="display: none" id="hiddenWrapper">
    <telerik:RadEditor ID="RadEditor1"  runat="server"></telerik:RadEditor>
</div>
<script>
    function f() {
        $get('hiddenWrapper').style.display = "block";
        $find("RadEditor1").onParentNodeChanged();
    }
    Sys.Application.add_load(f);
</script>