Firefox 31 has been released, so rm pre-v31 browser bug entry

docs/browser-bugs.html

@@ -47,12 +47,6 @@ lead: "A list of the browser bugs that Bootstrap is currently grappling with."
           <td><a href="https://bugzilla.mozilla.org/show_bug.cgi?id=975632">Mozilla bug #975632</a></td>
           <td><a href="https://github.com/twbs/bootstrap/issues/10690">#10690</a></td>
         </tr>
-        <tr>
-          <td>Firefox <strong>&lt;31</strong></td>
-          <td><code>position: relative;</code> doesn't work inside tables</td>
-          <td><a href="https://bugzilla.mozilla.org/show_bug.cgi?id=63895">Mozilla bug #63895</a></td>
-          <td><a href="https://github.com/twbs/bootstrap/issues/12467">#12467</a></td>
-        </tr>
         <tr>
           <td>Chrome</td>
           <td>Weird button behavior with some number <code>&lt;input&gt;</code>s</td>