When you visit a website in your web browser, it is accessed using either the HTTP or the HTTPS protocol. The HTTP protocol does not offer any encryption between your web browser and the web server where the website files are hosted. This is why the HTTP protocol is often subjected to the man-in-the-middle attacks where anybody including a hacker, WiFi operator or in some of the cases even the ISP can sniff the data transferred between your web browser and the website servers. So if you enter a password on a HTTP site, it is sent in plaintext over the internet to the web servers and can be stolen anywhere in the middle.
To counter this problem the HTTPS protocol is used by many websites. In the HTTPS protocol, end-to-end encryption is utilized so that the data transferred between your web browser and the web server stays encrypted throughout the middle route. Even if someone tries to steal somewhere en route to the web servers, they only get the encrypted gibberish.
If you want to make use of the encrypted HTTPS protocol in various websites, then you can install Smart HTTPS add-on which is available for Firefox, Chrome and Opera. What this add-on does is that it tries to switch to the HTTPS protocol for every URL you visit. But since not all sites offer HTTPS, when it fails to connect the URL through HTTPS, it silently reverts back to the HTTP protocol. This way you can make use of the HTTPS protocol for every possible URL that offers support for it, without actually having to find which URL supports secure URL and which does not.
And if there is a URL that must not be accessed via the secure connection (HTTPS), then you can just click on the Smart HTTPS lock icon and turn it off for that URL by clicking on Revert back to HTTP. For turning off the extension altogether, you can click on the big power icon. In the settings for the Smart HTTPS, you can see a list of all the URLs that you have wished to be accessed over HTTP. You can choose to remove them if needed.
Conclusion: For effortless and automatic switching to the HTTPS protocol for all the URLs that support it, you can use the Smart HTTPS extension for Chrome, Firefox or Opera.
You can get the Smart HTTPS extension for web browsers from http://mybrowseraddon.com/smart-https.html.