Change Firefox icon for different profiles

I have 2 profiles with firefox - default and local-browsing. “Default” as the name implies is the default profile that is launched with a new instance of firefox and local-browsing contains different settings is used for browsing of web apps on my LAN.

In order to create a shortcut for local-browsing, I copied the firefox .desktop file to ~/.local/share/applications/firefox-proxy.desktop and changed the following:

[Desktop Entry]
Version=1.0
Name=Firefox (Proxy)

...

Icon=firefox-nightly
StartupWMClass=firefoxProxy

However, only in the first few secounds when the application is launching does the taskbar show firefox-nightly, once the program is launched fully icon reverts to “normal” firefox. Is there something additional I have to do in order to retain firefox-nightly icon?