This probably is related to the URL "autofill" (inline URL bar autocomplete) feature turned on in Firefox 14. If you ever visited a secure page on npr.org, then when you type in npr in the address bar, Firefox will default to an HTTPS connection for other pages that the site never intended to be accessed that way. Not so convenient.
The simplest workaround is to disable autofill and instead choose matches from the autocomplete drop-down.
(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.
(2) In the filter box, type or paste autofill and pause while the list is filtered
(3) Double-click browser.urlbar.autoFill to toggle it from true to false.