removed obsolete known issue for Firefox (fixed in Firefox 64)

extension/ui/pages/help.html

@@ -469,12 +469,6 @@
 								a saved page, or to save a page stored on the filesystem.</li>
 						</ul>
 					</li>
-					<li>
-						Firefox
-						<ul>
-							<li>You cannot save selected tabs via the context menu because it is not possible to select multiple tabs.</li>
-						</ul>
-					</li>
 				</ul>
 			</li>