Add a warning about foreground processes in :h :!.

common/locale/en-US/various.xml

@@ -65,6 +65,10 @@
             the previous external command, so long as it is not preceded by a
             backslash and <o>banghist</o> is enabled.
         </p>
+        <note>
+            Do not run commands which require stdin or it will hang Firefox.
+            You can launch a process in the background (e.g. :! xterm &amp;).
+        </note>
     </description>
 </item>