Bug 1640153 - Added StartupWMClass to flatpak .desktop file r=mtabara DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D104309

taskcluster/docker/firefox-flatpak/org.mozilla.firefox.desktop

@@ -51,6 +51,8 @@ StartupNotify=true
 Categories=Network;WebBrowser;
 Keywords=web;browser;internet;
 Actions=new-window;new-private-window;profile-manager-window;
+StartupWMClass=Firefox
+
 
 [Desktop Action new-window]
 Name=Open a New Window