wrong icon appearing for desktop application on the dock/launcher

Ubuntu 17.10, gonme 3.26.2 (xorg)

I created the desktop application shown below. It appears on the desktop with the correct icon. When I start the application, it then appears in the launcher/dock - which is expected - but it uses the Firefox icon instead of the icon specified in the desktop file shown below.

Why is that? How do I get it to use the correct icon in the launcher/dock when it is started?

[Desktop Entry]
Version=1.0
Name=Firefox Web Browser
Comment=Browse the World Wide Web
GenericName=Web Browser
Keywords=Internet;WWW;Browser;Web;Explorer
Exec=firefox -no-remote -P ZohoCalendar --new-window --class="ZohoCalendar" %u https://www.zoho.com/calendar/signin.html
Terminal=false
X-MultipleArgs=false
Type=Application
Icon=/home/main1/applications/images/ZohoCalendar.jpg
Categories=GNOME;GTK;Network;WebBrowser;
MimeType=text/html;text/xml;application/xhtml+xml;application/xml;application/rss+xml;application/rdf+xml;image/gif;image/jpeg;image/png;x-scheme-handler/http;x-scheme-handler/https;x-scheme-handler/ftp;x-scheme-handler/chrome;video/webm;application/x-xpinstall;
StartupNotify=true
StartupWMClass=ZohoCalendar
Actions=NewWindow;NewPrivateWindow;
Name[en_US]=Zoho Calendar
GenericName[en_US]=Zoho Calendar
Comment[en_US]=Zoho Calendar

[Desktop Action NewWindow]
Name=Open a New Window
Exec=firefox -new-window
OnlyShowIn=Unity;
[Desktop Action NewPrivateWindow]
Name=Open a New Private Window
Exec=firefox -private-window
OnlyShowIn=Unity;