I am using FF nightly on Pop-os using gnome, got a problem when the FF nightly is pinned on to the dock, and when you open it another icon of the nightly will show up on the dock. So to fix this I need to add "StartupWMClass=Nightly" to the firefox-trunk.desktop. But when there is new update of the Nightly, it will overwrite the firefox-trunk.desktop losing my edits. Some reference on askubuntu. https://askubuntu.com/questions/975178/duplicate-applications-icons-in-ubuntu-dock/975230#975230