Computer Configuration > Administrative Templates > Firefox > Content Analysis (DLP)
Both
HKLM\Software\Policies\Mozilla\Firefox\ContentAnalysis
If this policy is enabled, you can specify a space-separated list of regular expressions that indicates URLs for which DLP operations will always be allowed without consulting the agent. The default is "^about:(?!blank|srcdoc).*", meaning that any pages that start with "about:" will be exempt from DLP except for "about:blank" and "about:srcdoc", as these can be controlled by web content. If this policy is disabled or not configured, the DLP agent will always be consulted.