The feature was available for some time, but not in such an elegant form
The third beta release in the new Firefox 23.x branch has been made available for download, by Mozilla, for all available platforms.The binaries for Mozilla Firefox 23.0 Beta 3 can be downloaded and run, without having to install anything.
The Mozilla developers have picked up the pace and they're going through the Beta versions almost on a daily basis.
This latest version introduces a few new features in the toolbox called Network Monitor. According to Mozilla, this is just a classic waterfall timeline view of the network activity.
A detailed list of upcoming features can be found on the official website.
Download Mozilla Firefox 23 Beta 3 for Windows
Download Mozilla Firefox 23 Beta 3 for Linux
Download Mozilla Firefox 23 Beta 3 for Mac OS X
Remember that this is a development version and it should NOT be installed on production machines. It is intended for testing purposes only.