Does Facebook only support HTTP or does it have HTTPS also?
As of July 2013, Facebook is https by default:
We now use https by default for all Facebook users. This feature, which we first introduced as an option two years ago, means that your browser is told to communicate with Facebook using a secure connection, as indicated by the "https" rather than "http" in [the URL]
However, the http interface remains
Some mobile phones and mobile carrier gateways don't fully support https. While we're working with the vendors of these products, we didn't want to leave https off entirely for affected users.
This is regrettable because it leaves possible the sslstrip attack whereby a man-in-the-middle rewrites https links to http, and serves a similar looking page at the http url. The solution is the HSTS header which tells the browser to always use https for a given site.
Yes - have a look at https://ssl.facebook.com/, however I do not think it is intended for use. All links from this site will redirect you back to non-SSL. I suppose it's grand if you want to check your live feed securely.
As a note: Going to https://www.facebook.com will throw a certificate error, so rather use the above link for access.
That Firefox add-on will force SSL connections for Facebook, in addition to a growing number of other websites that support SSL (like wikipedia, google, meebo, etc). Future add-on updates will add support for additional sites.