Linked from the Linuxiac post:
Mozilla’s Phabricator revision D277804
The proposed change just stops setting middlemouse.paste to true by default, and there are comments suggesting tying it to GTK's corresponding preference.
GNOME merge request: Disable primary-paste by default
People that know about this functionality and really
love this functionality can easily override the
setting.
gsettings set org.gnome.desktop.interface gtk-enable-primary-paste true