CORS Everywhere is a Firefox addon designed for developers to manage Cross-Origin Resource Sharing (CORS) effectively. This subscription-based tool allows users to alter HTTP responses to enable CORS, making it easier to test and develop web applications without the typical restrictions imposed by browsers. The addon operates as a WebExtension, ensuring compatibility with Firefox while maintaining browser security protocols.

Users can toggle the functionality through a customizable button in the toolbar, which indicates its status with color codes. CORS Everywhere supports various options, including enabling the addon at startup and configuring an activation whitelist for specific URLs. It is essential to note that while the addon modifies requests, it does not compromise the inherent security features of Firefox. This makes it a valuable tool for developers needing to conduct CORS tests and manage responses efficiently.