Since upgrading to FireFox 39, I can't read digitally signed emails. Outlook tells me that I need to install S/MIME. I download S/MIME and click to install, then restart the browser as instructed. FireFox indicates that S/MIME 1.0 is enabled. But I still can't read any emails that are digitally signed. All that is displayed is the header portion (who the email is from). The emails continue to say either: (1) "S/MIME isn't supported in this view. Click here" - to pop out a separate window; or after opening a separate window for the digitally signed email: (2) "This message has a digital signature, but it wasn't verified because the S/MIME control is out of date. To install the latest version, click here."

I've repeated the attempted install of S/MIME several times, to no avail.