It seems that if you load a site in a new tab, Firefox will try to establish a connection and the tab's status will be set to "Loading...", but during that process and upon a failure to do so the Address Bar will not show the URL of the site. This is hard to reproduce because you need a non-working site (whose DNS resolves). I found out that with invalid URLs or sites that don't DNS resolve you get an error message and the URL is displayed, but for this particualr case, you get no errors and the URL isn't displayed. I've had this happen on this site: http://www.kdedevelopers.org/node/view/1129 but once it's back up I don't think that you'll be able to reproduce it that easily. Pinging www.kdedevelopers.org while it did not load in Firefox responded properly, so I think that Firefox is waiting for the HTTPD (which is probably busy at the time). I think that it is important to have the URL show up in the Address Bar because failure to do so makes it very difficult to relod the page!
*** This bug has been marked as a duplicate of 231393 ***