Smart HTTPS от ilGur

Smart HTTPS is a Firefox extension that helps you always use the secure HTTPS protocol, if supported by the site’s server. It automatically changes HTTP protocol to the secure HTTPS, and if loading errors occur, the extension will revert back to the original HTTP protocol.

HTTP vs. HTTPS — why it matters
HTTP, or Hyper-Text Transfer Protocol, is the way a server communicates with the browser. HTTP enables visitors to view a website and send information back and forth to the server. HTTPS, or Hyper-Text Transfer Protocol Secure, is communication through a secure connection. Communications through HTTPS are encrypted by a secure certificate known as an SSL. The encryption prevents third-parties from eavesdropping on communications to and from the server. Read more about HTTPS here.

Key features
a. Automatically change all HTTP requests to the secure HTTPS.
b. All websites (addresses) that do not support HTTPS are remembered in order to speed up the protocol-check process. Note that you can see and delete these items from the extension's Options page.
c. Toolbar button serves as the extension’s ON-OFF switch. Blue indicates ON and Gray is OFF.
d. Add desired websites to the whitelist (see Options page).

Note: to report bugs, please visit the extension homepage and find the bug report form.