How can we stop Opera from usurping /usr/bin/gnome-www-browser?

While Firefox is of course my browser of choice, I have a few others, for reasons, including Opera. In XFCE's "Settings Manager", in "Default Applications", Firefox is duly registered as the "Web Browser". So far, so good, everything works as expected. Now, it sometimes happens that an indirect opening of a web page results in the uncalled-for opening of Opera, for example when running debian-reference. No big deal, I just adjust sensible-browser via update-alternatives --config gnome-www-browser, and the deed is done … for a while. Then, I find Opera again as the sensible-browser, even though XFCE's "Default Applications" remain unchanged and Opera *is not* set as the default browser as per its own settings. I have https://deb.opera.com/opera-stable/ in my software repos, and automated updates, so that *may* be the route of this unseemly behavior.