Previous versions of Firefox used to have a padlock icon in the address bar whenever you browsed to a HTTPS site.
Since Firefox 4 it no longer seems to be there. What happened to it? Is there any way I can get it back?
It was removed in Firefox 4 - the intention now is that the site's security information is presented in the Site Identify Button. You can add a similar concept of a padlock back to the Site Identity Button with the Padlock addon.