HTTPS Finder automatically detects and enforces HTTPS connections when available. It also provides one-click creation and in-browser editing for HTTPS Everywhere rules.
Here is a little more about the add-on from the developers:
The extension sends a small HTTPS request to each HTTP page you browse to. If there is a response, the certificate is checked for validity (any certificate errors will result in no notification, and no further detection requests during that session). If valid, HTTPS is automatically enforced (can be disabled for an alert only, with no redirect), and the user is given an option to save the auto-generated rule for HTTPS Everywhere. Version 0.51 adds “in-Firefox” editing of the rule before saving.
If you are worried about your online security, this is a great add-on to have. You can pick up HTTPS Finder from the Firefox Add-ons website.