
Firefox automatically creates a scheduled task to check whether it's the default browser, and warn you if another browser attempts to change that. If you wish to disable this feature:
- Open Firefox > Type about:config > Accept the warning.
- Type default-browser-agent.enabled > Change to "false".
- If already exists, disable or remove the task from Windows Task Scheduler > Mozilla.
*If you don't change the setting within Firefox, the task will get re-created when Firefox is updated or re-installed.