My yahoo mail is not secure on the firefox browser.
Fx89Padlock-Triangle: A padlock with a triangle indicates that Firefox is not blocking insecure passive content, such as images. By default, Firefox does not block mixed passive content; you will simply see a warning that the page isn't fully secure. Attackers may be able to manipulate parts of the page like displaying misleading or inappropriate content, but they should not be able to steal your personal data from the site.
This is likely caused by emails with insecure remote content. In this case, this is not at fault of Firefox. If you are worried, you can enable HTTPS only mode in Firefox, with the instructions found at HTTPS-Only Mode in Firefox