In Bug 1699892 (701d0e6960e3aeede92e32b728b880adca41c76b), Mozilla introduced `svg.context-properties.content.allowed-domains` as a list of domains where non-standard, SVG context paint keywords are allowed. We should set this pref-list as empty. Context paint is already allowed (and used) within Firefox, for local resources, but exposing it to web content opens a new vector of possible SVG-based exploitation.