The HTTP Nowhere extension adds a button to your Firefox toolbar that lets you disable all unencrypted web requests and only permit those you choose. When URLs are blocked, an indicator appears on the button and you can then choose to allow or ignore them in the future. You can also manually manage your allowed and ignored URLs and quickly switch between enabled (red) and disabled (gray) mode.
Here is a little more about HTTP Nowhere from the developers:
One of the most effective ways to improve your privacy on the web is to opt for encrypted communication whenever possible. By blocking plain HTTP web traffic by default, this extension gives you the power to decide exactly which unencrypted URLs, if any, you will permit your browser to make requests to on your behalf. HTTP Nowhere has been tested with and works when HTTPS Everywhere is installed. However, when both extensions are enabled, HTTP Nowhere effectively disables HTTPS Everywhere’s automatic redirecting capability.
You can pick up HTTP Nowhere from the Firefox Add-ons website.