Bump supported Firefox to 4

Range.getBoundingClientRect is not supported in FF 3.x

index.html

@@ -185,7 +185,7 @@
   in <em>standards mode</em> (HTML5 <code>&lt;!doctype html></code>
   recommended) are supported:</p>
   <table style="margin-bottom: 1em">
-    <tr><th>Firefox</th><td>version 3 and up</td></tr>
+    <tr><th>Firefox</th><td>version 4 and up</td></tr>
     <tr><th>Chrome</th><td>any version</td></tr>
     <tr><th>Safari</th><td>version 5.2 and up</td></tr>
     <tr><th style="padding-right: 1em;">Internet Explorer</th><td>version 8 and up</td></tr>