I see, so I could remove it, but wouldn’t I still see the same behavior, since firefox nightly apparently also uses the firefox window class?

Hmm, but I thought the StartupWMClasses in the desktop files are correct? I thought the issue is that firefox-nightly uses the firefox class in addition to the firefox-nightly class

The desktop files are named firefox.desktop and firefox-nightly.desktop, both in /usr/share/applications.

EDIT: For completeness, I also checked the window classes of the regular firefox:

It can be observed that it uses only the window class firefox, but twice?

I just checked the .desktop file for firefox nightly. It contains only firefox-nightly as its StartupWMClass. So I went ahead and also checked the window class firefox nightly is running as and found the following:

Does this mean that firefox nightly is using both, the firefox and the firefox-nightly window classes?

I use Firefox nightly and recently switched to Wayland (KDE). Ever since I switched, the taskbar often displays Firefox nightly with the regular Firefox icon, and the name Firefox instead of Firefox nightly. After reading your blog post I’m thinking that the culprit could be an incorrect StartupWMClass. But I don’t get why this issue never showed on X11 then.