To confirm, you MUST be able to see the pop-up with the name suggestion for the mention mod to work. If you don't see this, the member will not be pinged. You can identify whether it has worked when you publish your post. If the username is not in bold and doesn't link to the member profile, they won't have been pinged.
Next up is to figure out why you guys are not seeing that pop-up.
When you type slowly, just @fran for example, does nothing show at all?
@Francesca: I am on Mac 10.6.8 using firefox 48.0.2 and cannot upgrade from either.
It's possible that this is the issue as that Firefox is out of date by some years, but it might be something else so I will continue to try.