Are you getting this on Gmail? "Warning: unexpected msg classname:X0uMP"

"Warning: unexpected msg classname:X0uMP"

I am a Gmail user and I have been getting this message quite frequently whenever I get an attachment on Gmail.

A bit of Googling showed me that this is connected to Gmail S/MIME add-on in Mozilla Firefox browser.

Solution:

In Firefox, Click "Tools"on the main menu and select "Add-ons".

Then click "Extensions" tab. Then click on “Gmial S/Mime - Send and receive signed and encrypted messages in Gmail”.

Then click “Find Updates” button at the bottom.

There should be an update. Go ahead and update the Add-on.

That should correct the annoying warning and save you several mouse clicks too.