"Firefox” will damage your computer. You should move it to the Trash.
Get this error on my MacBook Pro running El Capitan when trying to open Firefox. It had previously been working fine.
You are saying a notice comes up saying this? Is it a separate message, or is Firefox displaying it?
In order to better assist you with your issue please provide us with a screenshot. If you need help to create a screenshot, please see How do I create a screenshot of my problem?
Once you've done this, attach the saved screenshot file to your forum post by clicking the Browse... button below the Post your reply box. This will help us to visualize the problem.