Doing a recursive grep under /usr, the only file containing this particular URL is /usr/lib/firefox/ubuntumate.cfg, shipped by the ubuntu-mate-default-settings package.
Attempting to purge this package says it'll install debian-mate-default-settings instead, and remove mate-core, mate-desktop-environment-core and a few others as well. So apparently this package is needed in order to have Mate available as an alternate desktop environment.
Doing a recursive grep under /usr, the only file containing this particular URL is /usr/lib/ firefox/ ubuntumate. cfg, shipped by the ubuntu- mate-default- settings package.
Attempting to purge this package says it'll install debian- mate-default- settings instead, and remove mate-core, mate-desktop- environment- core and a few others as well. So apparently this package is needed in order to have Mate available as an alternate desktop environment.