The Mozilla Foundation’s latest version of Firefox, version 34, has started running searches when you type go/mail into the address bar without prefixing with httpd:// or using the fully-qualified name go.middlebury.edu. This follows similar moves by Chrome and Safari to prefer searches over host-name lookups when there isn’t a fully-qualified domain name in the URL bar.
The good news is, that confirming the dialog to go to GO instead of to search will set a preference allowing GO shortcuts to continue to work:
If you forget to click the confirmation, it is possible to manually add the preference to make GO links work permanently. See the LIS Wiki for more details.