The steps to fix it are these, and I cite the original web page further below: If you understood that last sentence, then it means you've probably done it on your computer I know I have. Today this happened again with no ability to add an exception, and after looking at various websites for hours, trying to find a fix, I stumbled on one: Having Firefox '.automatically search for and import CAs that have been added to the Windows certificate store by a user or administrator.' These are root certificates which you've manually imported.
Unfortunately, it does not give you the option to add an exception, which clears up the problem most of the time. Periodically when Firefox is updated, it starts rejecting Yahoo Mail from loading, giving the 'Your connection is not secure' / SEC_ERROR_UNKNOWN_ISSUER error.
