So to fix this, I wrote a simple script called it .firecal and saved it in my home directory. The script has one line
I made the script executable:
~$chmod +x .firecal
Then in a terminal run :
~$gsettings set org.gnome.desktop.default-applications.office.calendar exec /home/user/.firecal
Now when I click on the open calendar FireFox opens up Google calendar.