How about the main page at

https://www.yahoo.com/

Do you get that error there?

There has been a problem with

https://my.yahoo.com/

this morning for many users but hopefully that is clearing up now.

Clear the Cache and remove the Cookies from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove the Cookies" from websites that cause problems.

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"