Since I upgraded from Ubuntu 17.10 to Ubuntu 18.04 whenever I open Thunderbird there is a globe icon, and sometimes also headphones icon, appearing in my list of new emails.
These icons hide the titles of my emails. When I move the mouse over the icons they partially disappear. When I scroll down there is another globe icon further down the list.
Solution: : Installing the fonts-symbola package fixes the problem.
sudo apt-get install fonts-symbola