It sounds like a session management issue. Click to check the configuration and ensure the payment gateway redirect is properly handled in Magento2. Feel free to provide more details for further assistance.
Session-related problems can often be browser-specific, and in this case, it appears to be impacting Firefox. To troubleshoot, I recommend checking the cookies and session settings in both Magento and Firefox. Ensure that your Magento installation is configured correctly for session handling, and review any customizations or extensions related to sessions. In Firefox, check its cookie and privacy settings to ensure they are not overly restrictive, preventing the preservation of session data. Clearing browser cache and cookies in Firefox might also help resolve any caching-related issues. If the problem persists, consulting with Magento support or your payment gateway provider for browser-specific recommendations and potential solutions could be beneficial.