Why can't I make this app a favorite, in the dock?

Question

I'm trying to make Waterfox (a Firefox fork) a favorite in the dock, but the option isn't there. I can't do it by dragging the icon to a different position, either. It worked fine until Waterfox updated recently. What gives?

Answer 1

This can happen when the property StartupWMClass is not set in the corresponding .desktop file. Open the .desktop file you're trying to set as a favorite in a text editor, and add (in the [Desktop Entry] section) a line saying:

StartupWMClass=[see below]

To determine the correct value, launch the application (i.e. by double clicking the .desktop file). Now open a terminal and run xprop WM_CLASS. Click on the application. The terminal will print something like

WM_CLASS(STRING) = "Navigator", "waterfox"

Take the most "specific" seeming value and put it, without quotes, as the value of StartupWMClass in your .desktop, e.g. StartupWMClass=waterfox.