What does a lock sign with a red line diagonally going through it mean?

Question

When I use Firefox to go to the yahoo mail login page, I get a lock with a red line diagonally going through it in the address bar. But when I refresh it the red diagonal line goes away.

Is it secure when the red line appears? Why is this happening?

This happened a while ago, but isn't happening now.

Answer 1

red line means its not locked.no line then its locked and safe.

Answer 2

this is just a security heads up. its telling you that you are passing an encrypted area before you connect to the yahoo server. just in case run a virus scan if you have one.