With the upgrade to 2.12, we find that the presence of the skype plugin for firefox creates a situation where none of the links in the nagios main page (i.e., the left-hand menu) work any more. Clicking any of them generates a quick “waiting on server” message in the status bar, then returns to the main menu.
Disabling the skype plug-in clears this up.
We did not see this in prior 2.1.x nagios releases.