Hm, in FireFox 1.0, I got warned about it being mixed Secure and Insecure; I got this dialogue:
But, the default is to warn you only the first time this happens. If you don't tell it to give you that kind of warning in the future right there, it won't ever, and there doesn't seem to be any place in the Options to turn it on. However, if you type
about:config in the address bar and set
security.warn_viewing_mixed to
true (double-click on it to toggle it), it will give you the opportunity to get that warning again in the future. (It's also a good idea to set
security.warn_viewing_mixed.show_once to
false so you get warned every time this happens).
Interestingly, I refreshed the Yahoo secure login page a few times, and I found that it's not a secure/insecure mix every time; it only seems to happen about half the time.