The "leader ERROR" you are referring to is a known issue in Gecko and usually occurs
contexto when you are running multiple Gecko instances at the same time.
To fix this error, you can try the following steps:
- Close all instances of the Gecko browser.
- Open the Task Manager on your computer (Ctrl+Shift+Esc on Windows, Command+Option+Escape on Mac).
- Look for any running Gecko processes and end them.
- Open the Gecko browser again and check if the error still persists.