Seems like people agree in the Mozilla thread, that they should simply follow gtk (if they change it): https://phabricator.services.mozilla.com/D277804
GNOME/gtk on the other hand is apparently all for it (and immediately restricted the discussion to project members): https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/-/merge_requests/119
My favorite argument: Because Linux has less marketshare than Windows and OSX, more users (wo might never use Linux (or GNOME)) are not expecting the middle-mouse-paste, so it should be disabled by default...