I have it installed manually and manually update directly from mozilla. Here is mine, which you would have to edit with a text editor. I have mine in /home/my user name/.local/share/applications/ I would think that yours should be in /usr/share/applications if you installed via pamac. In xfce panel, you can create a launcher, point to the executable and chose an icon from the firefox nightly installation in firefox nightly/browser/chrome/icons/default/ I am not a gnome user so I cannot comment on how to achieve this in gnome.
Exec='/home/my user name/Software/firefox nightly/firefox'
Icon=/home/my user name/Software/firefox nightly/browser/chrome/icons/default/default128.png
Path=/home/my user name/Software/firefox nightly
yes my user name=jrichard if that is what you are talking about, I just put my user etc for demonstration purposes. If I click on the .desktop file in local/applications, I get the untrusted warning, launch anyway, blah , blah, but it works.