When you open a website in your web browser, your browser fetches it from the web server over either the HTTP or the HTTPS connection. The extra S in the latter stands for secure. Using HTTPS for accessing web sites means that any data transferred between your web browser and the web browser is encrypted. HTTPS secures the connection and prevents any eaves dropping by anyone at any point in the complex route through the internet between your computer and the web server. This is why when accessing websites to carry out financial transactions or transfer sensitive information, you should check whether it is being accessed via the HTTPS protocol.
You can easily tell if a web site is being accessed through HTTPS by looking at the website address in the web browser. But to make sure that you use HTTPS whenever possible, you can use the Smart HTTPS extension in the Firefox web browser. What this extension does is that it automatically loads all the websites through the HTTPS protocol, but if the website is not configured to use the HTTPS, then it reverts backs to HTTP. The switching to HTTPS and back to HTTP is done so fast that you won’t even notice it.
You can click on the blue lock icon in the Firefox toolbar to force it to use HTTP or HTTPS protocol for a website. So even if a website is being accessed with HTTPS, you can make it access through HTTP by white-listing it if you need that perhaps because of compatibility reasons. Similarly, you can make any website load with HTTPS only by adding it to blacklist.
In the settings for the Smart HTTPS, you can change the time it should wait for the secure connection to work. By default it waits for 3000 ms (or 3 seconds) for the HTTPS to work and then auto-reverts to HTTP. This may not be enough when you are trying to login to certain websites and you may want to increase this time limit. Other options include enabling it in incognito mode, reverting to HTTP if any resource is loaded through HTTP and more.
Conclusion: Smart HTTPS is very useful extension for the Firefox users and can automatically switch to the secure HTTPS connections whenever possible. You can make it enforce HTTPS even when you are using the incognito mode.
You can get the Secure HTTPS extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/smart-https/.