I've been at this for hours now. Nothing works, .desktop file below. I don't know why, but I can't drag&drop the open firefox-dev icon to my favorites, only when I look for it in Applications windows and drag it from there. But that way it just opens a new icon instead. Before this issue gets closed for a mildly similar question, below are all of the solutions I've tried.

[Desktop Entry]

Version=1.0

Name=Firefox Developer Edition

StartupWMClass="firefox-aurora", "Firefox Developer Edition"

GenericName=Web Browser

Exec=/opt/firefox/firefox %u

Terminal=false

Icon=/opt/firefox/browser/chrome/icons/default/default128.png

Type=Application

Categories=Network;WebBrowser;Favorites;

MimeType=text/html;text/xml;application/xhtml_xml;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;

X-Ayatana-Desktop-Shortcuts=NewWindow;NewIncognito

I've tried:

Cannot add custom launcher to Ubuntu Dock ('Add to Favorites' option does not show up)

Ubuntu Dock adds a new icon when I open certain programs

Duplicate icons for manually created GNOME launcher items

How do I install the Firefox Developer Edition?

Firefox developer edition icon duplication in Gnome shell