User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:58.0) Gecko/20100101 Firefox/58.0 Build ID: 20171105133105 Steps to reproduce: On the latest firefox-trunk package from the PPA, 58.0a1 (2017-11-05) (64-bit), the firefox-trunk.desktop file doesn't specify a WM_CLASS. Actual results: Because of this, Nightly uses up two icons on the gnome dash/dock. One that has the appropriate icon, but never has windows, and another that is of WM_CLASS "Nightly" and can't be added to favorites because it doesn't have a desktop file of its own. Expected results: A quick fix to this is to add StartupWMClass=Nightly to /usr/share/applications/firefox-trunk.desktop: all the windows are now in the same class, and thus only use one icon.
If you keep WM_CLASS as firefox, you can use --class command line option. Also, this issue should fix firefox-trunk.desktop. So we should file a bug to Ubuntu PPA team.
s/we should file a bug to Ubuntu PPA team/you should file a bug to Ubuntu PPA team for Firefox package/