If you encounter this error message as often as I do, you will thank your various gods for this extension, which allows you to hide the warning message on a per site basis, for both Firefox as well as Thunderbird.
Here’s the default Firefox warning message… so irritating.
And here’s the same warning message once the extension is installed. This is the last time I saw this error for this particular bookmark.
The extension will also allow you to hide the message for expired certificates as well. You can delete the remembered domains in the extension’s options panel.