Ubuntu 21.04 custom firefox.desktop creates new icon rather than run indicator by custom icon

We share a desktop so I have a Firefox application launcher for each of us to load our individual profiles; here's mine:

[Desktop Entry]
Version=1.0
Name=s3rvant-Firefox
Comment=Browse the World Wide Web
GenericName=Web Browser
Keywords=Internet;WWW;Browser;Web;Explorer
Exec=firefox %u -P s3rvant --class s3rvantfox
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=/home/s3rvant/Launchers/Firefox/Firefox-s3rvant.png
Categories=GNOME;GTK;Network;WebBrowser;
MimeType=
StartupNotify=true
Actions=NewWindow;NewPrivateWindow;ProfileManager;

[Desktop Action NewWindow]
Name=Open a New Window
Exec=firefox -P s3rvant --class s3rvantfox -new-window

[Desktop Action NewPrivateWindow]
Name=Open a New Private Window
Exec=firefox -P s3rvant --class s3rvantfox -private-window

[Desktop Action ProfileManager]
Name=Open the Profile Manager
Exec=firefox -ProfileManager --class s3rvantfox

In Ubuntu 20.04 this has worked fine keeping the various profile window indicators grouped on their respective launcher icons as expected. However since upgrading to 21.04 when I click one of these launchers the default Firefox icon is added to the dock for the newly opened window with running indicator by that icon. Clicking on a 2nd profile opens another window with indicator also by that default icon even though the 2nd window is using the 2nd profile.

I've tried adding the following to top block but no change:

StartupWMClass=s3rvantfox

I've also tried moving the launchers from ~/.local/share/applications to /usr/share/applications but again no change.

Any idea how to fix?

I believe that I have the solution, though it might require some back-and-forth until it actually works. Anyway I had a very similar problem (I had such a link in the favorites, which opened a new icon), and it did at the end work for me!

Key is the solution provided in the Stackoverflow entry on duplicate icons in the navigation.

I'm not sure whether I need to/should copy the solution from there into here (should I? comments on that?), so I'll just refer to it (solution by pomsky) and gave an abstract repetition of that solution plus additional comments.

• Indeed the launcher must be copied into ~/.local/share/applications
• There you will indeed have to add the line StartupWMClass=VALUE -- however it MIGHT be that the value that you set there is not the entirely correct one. You will fond out while value that is by running that program (i.e., open you link), run xprop WM_CLASS in a terminal, and then click on the program. It will then display the correct value.
• If that doesn't work, try adding it to the favorites (after you copied the edited file into the above-mentioned folder).