Global Privacy Control

Global Privacy Control (GPC) is a proposed specification designed to allow Internet users to notify businesses of their privacy preferences, such as whether or not they want their personal information to be sold or shared. It consists of a setting or extension in the user’s browser or mobile device and acts as a mechanism that websites can use to indicate they support the specification.
— Global Privacy Control

• Global Privacy Control
• Participating Organizations

Technically, it is almost identical to the already existing Do Not Track header. DNT is mostly just ignored in the industry, though. That is why the Global Privacy Control's aim is to make adhering to GPC legally binding.

GPC is intended to serve as an expression of users’ intent to invoke their online privacy rights. Depending on the jurisdiction and applicable laws, a user’s expression through GPC may have legal impact. However, GPC on its own does not create any legally binding obligations.
— Global Privacy Control

Firefox currently does not support Global Privacy Control yet. But it is enabled by default in the Privacy Badger addon by the EFF.

• [Bugzilla] Support GPC / globalprivacycontrol.org Signal
• Privacy Badger addon