The latest stable version is always available from Mozilla's Add-ons.
Bleeding-edge releases are usually made available on Launchpad and early-access builds are also on our GitHub releases page.
List of current issues is available at our bug tracker.
Help & feedback are both appreciated! ;-)
You can use make-xpi.py script to build xpi and clean temporary files:
$ ./make-xpi.py [TARGET]
$ python3 make-xpi.py [TARGET]
Available targets: all, theme, extension and clean. Default is all.
For more details please check this page.
Default:
With some tweaks, made available by GNOME Theme Tweak, Firefox can even look close to native on GNOME 3.12+: