Using :

• Moodle 2.6 (build 20131118)
• Standard theme
• Language en
• and a site admin user

with both latest version of Firefox (25.0.1) and Chrome (31.0.1650.57 m) when we try to go to Site administration under the Administration block, the sub menus won't load. The arrow icon turn blank and the text "Site administration" is no longer clickable. I've got this line on the console but no error:

POST http://[....]/lib/ajax/getsiteadminbranch.php

and a response that seem to have all the menus I should see:

???{"id":null,"name":"Site administration","type":71,"key":"root","class":"type_siteadmin root_node","icon":{"component":"moodle","pix":"i\/navigationitem","alt":"","classes":["smallicon","navicon"],"title":""},"title":"","hidden":false,"haschildren":true,"children":[{"id":null,"name":"Notifications","type":70,"key":"adminnotifications","class":"type_setting","icon":{"component":"moodle","pix":"i\/settings","alt":"","classes":["smallicon"],"title":""},"title":"","link":"http:\/.........

It work fine however with IE8.

Note that a user with Manager rights work fine, it seem only to occur with a site administrator.