I do not want to use Google Chrome - I purposely installed Chromium. But whenever I click on a link in an external application ( like a link in a pdf or email) Google chrome launches. It is not in installed applications in software center
Go to System Settings --> System Info --> Default Applications. Then choose Firefox (or another browser you like) at the point Web.
I hope I could help ;)
Run the below command to switch your default browser
sudo update-alternatives --config x-www-browser