about:config is the advanced configuration for Firefox. Just type it in the address/URL bar to edit it.
If you want to be able to see the entire address in the URL bar, you will need to set the browser.urlbar.trimURLs setting to false. This will allow you to select and copy just the server address in the address bar and not get the unseen 'http://' portion of it.
If you want to keep Firefox from assuming you want to go to some specific address you went to previously, you will need to set browser.urlbar.autoFill to false. This is annoying if you want to go to Facebook or YouTube. You type the address, and Firefox decides you want to go to some specific picture or message within Facebook. This is also very annoying when you type IP addresses.