How can I get Firefox to prefer HTTPS over HTTP?

For example, I type superuser.com in the browser bar of Firefox, but it automatically goes to the HTTP site. I want the HTTPS site by default.

Another alternative is HTTPS Everywhere. It's available for Firefox, Chrome and Safari.

Since it is developed from the collaboration between EFF and the TOR project, I tend to believe this plugin more.

It's also open source and available under GPLv3 license.

Firefox addon "HTTPS by default" works: https://addons.mozilla.org/en-US/firefox/addon/https-by-default/?src=ss

Well, you can't do this automatically, there is no option in Firefox, like browser.urlbar.*, for this.

So you either use one of the browser extensions mentioned in the other answers here, or as I'd suggest (avoiding usage of browser extensions you don't really need), just make sure that you bookmark https://superuser.com instead of http://superuser.com.

Now you can just type superuser.. in the browser bar and find the right link.

You can also restrict the suggestions in the url bar by using specific characters, for example, using * somtehing (Asterisk) only finds matches in your bookmarks.

More examples: here

One more thing about HTTPS Everywhere: In addition to the issue with requiring predefined rules (as mentioned here in this comment), there is (or was, maybe they adressed this in the meantime) also the problem that HTTPS Everywhere saves the user preferences into the prefs.js inside your Firefox profile directory.
So you end up with a lot of user_pref("extensions.https_everywhere.SITE, BOOL); lines in your prefs.js, maybe even thousands of entries that increase the size of this file and slows down load time. Not to mention possible privacy issues.