Opera browser set itself as default without asking!

I just downloaded opera-stable_56.0.3051.99_amd64.deb from https://www.opera.com/de/download and ran sudo dpkg -i opera-stable_56.0.3051.99_amd64.deb It complained about some missing dependencies so I ran sudo apt-get -f install sudo apt install apt-transport-https then reran sudo dpkg -i opera-stable_56.0.3051.99_amd64.deb anyway, am worried about what evil that has done to my otherwise clean debian distribution and how to undo it. At the least it has replaced the firefox which used to launch from the icon at the bottom of xfce with itself. Actually, the command under the button is: exo-open --launch WebBrowser %u ..so no opera keyword. How do I get it back to how it was and if necessary clean up anything else bad which it might have done?