HTTPS (HTTP over SSL or HTTP Secure) is the use of Secure Socket Layer (SSL) or Transport Layer Security (TLS) which is a sub layer under HTTP application layering. HTTPS encrypts and decrypts webpage requests which ensures having secure sessions on websites e.g. Facebook. This can allow having a safe browsing session which can bar hackers or malicious programs from compromising your system. HTTPS Finder is a Firefox extension that automatically detects and alerts when a secure HTTPS connection is available for a website. You can also enable automatic redirection to make sure that you are automatically directed to a secure HTTPS version of a website (when possible).
When you visit a website that has a secure version available, you get a prompt if you would like to switch to the secure version.
You can also enable automatic HTTPS switching from Tools -> Add-ons -> Extension ( HTTPS Finder Options) and disable prompts for partially encrypted pages.