Hello, Thank you for reaching out to the Firefox Support Forum. I'll be happy to assist you with your question. Regarding the black dot that briefly appears on the left or right margin of the active Firefox window, this behavior could be related to various factors, including your operating system, browser extensions, or other software running on your computer. To help troubleshoot the issue, I would recommend the following steps: Check for browser extensions: Disable any installed browser extensions one by one and observe if the black dot still appears. This can help determine if an extension is causing the issue. Test in Safe Mode: Restart Firefox in Safe Mode to temporarily disable all extensions and themes. If the black dot doesn't appear in Safe Mode, it's likely that an extension or theme is causing the behavior. Update Firefox: Ensure you have the latest version of Firefox installed. Sometimes, updating to the latest version can resolve unexpected behavior or glitches. Clear cache and cookies: Clearing your browser cache and cookies can help eliminate any stored data that might be causing the issue. If the issue persists or you need further assistance, please provide additional details such as your operating system version, any specific extensions or themes you have installed, and any recent changes you've made to your system. This information will help us investigate the issue more effectively. I hope these steps help in resolving the issue you're experiencing. Feel free to provide more details or ask any further questions. We're here to assist you. Best regards, Suram.