Ever since Firefox 4 got released, users had to install an extension to bring back the status bar on the bottom as well as its progress meter. Now that this is no longer an option ever since Quantum got released (since you can no longer add your own toolbars and elements to toolbars), there's currently no way to bring back this part of Firefox 3.6's UI to the latest version. I still very much prefer the text being in the status bar plus the clearer indication of loading progress with the progress meter rather than small popups on the bottom each time a page is loading.
HTML has a progress meter available as a tag and Firefox has implemented it way back in version 6: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/progress