How can I make a desktop launcher for a specific Firefox profile?

Isn't it just firefox -p [name of profile]?

We share a single PC account and do exactly this to easily separate our profiles; here's what I have for my launcher for example:

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

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

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

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

Here I'm using a custom icon to help tell our profiles apart on our dock and use a custom window class so each profile's windows will group with each other as well.

firefox --help ?

Fwiw: there us a Gnome extension to add profiles to the top bar

The list of possible CLI parameters can be found in the Mozilla wiki https://wiki.mozilla.org/Firefox/CommandLineOptions