The shortcut TAB key provides correct behavior only for IE (  entities are added). UNder Firefox and Chrome the behavior is incorrect and not expected.

In FF - nothing is added to the content, moves the focus to the next HTML element.

In Chrome - a SPAN element is inserted.

A possible resolutions replacing the added shortcut for TAB key with the InsertTab command:

<telerik:RadEditor ID="RadEditor1" runat="server" OnClientLoad="OnClientLoad">
</telerik:RadEditor>

<script type="text/javascript">
	function OnClientLoad(editor, args) {
		var shortcutManager = editor.get_shortCutManager();

		if (shortcutManager.findShortCutByName("InsertTabMozilla")) {
			shortcutManager.removeShortCut("InsertTabMozilla");
			editor.addShortCut("InsertTab", "TAB");
		}
	}
</script>