I contacted support, and got timely and valuable help.
The solution was to clear Firefox cache, and LastPass cache, then log off (I did not log off, but rebooted the computer, which anyway forced the login since the cache was empty.)
Also, I realized that I was restrictive on cookies with the content protection. I was set to: "Cookies from visited web site" which might have triggered the problem, but I cannot confirm that.
Extract of the interesting section from the Customer Support reply:
Kindly try to clear cache and refresh the vault using the steps below:
1.Clear local cache - Click on LastPass icon > Account Options > Advanced > Clear local cache
2.Refresh sites - Click on LastPass icon > Account Options > Advanced > Refresh sites
3.Clear browser cache - http://www.wikihow.com/Clear-Your-Browser's-Cache
4.Logout, log back in and test the issue again.
We are here to assist our customers 24 hours a day Mon-Fri.