Installation Steps

1. Open Firefox

2. When you view any website for example Facebook.com if itz not encrypted from server side it will appear in the address bar as HTTP

3. Go to the website https://www.eff.org/https-everywhere and click https-everywhere.xpi from the attachment which is at bottom part.

4. Firefox will ask you to install the software as in the image below

5. click Allow (which is on the left corner)

6. Then you will get the following screen

7. Select Install

8. The HTTPS Everywhere Addon will be installed and Ask you to restart the firefox browser

9. This window will appear as you restart the browser

10. Close this window and try the website you viewed before installation.

11. You will now observe the website URL is HTTPS (Encrypted) instead of being HTTP when you checked it before

 Your Firefox Browser is now safe from Firesheep attack.

Note:- This comes with a predefined list of websites that will force the website encryption. By installing the plugin you are automatically safe on the supported websites.
Default list: Amazon, DuckDuckGo, EFF, Facebook, GMX, Google Search, Google APIs, GoogleServices, Identica, Ixquick, Live, Mail.com, Meebo, Microsoft, Mozilla, NL Overheid, NYTimes, PayPal, Scroogle, Torproject, Twitter, WashingtonPost, Wikipedia, WordPress.com, GentooBugzilla, Noisebridge, Zoho.
To add more to the list : https://www.eff.org/https-everywhere/rulesets