While trying to read a different email, the screen fades and a message appears at the top "Mozilla Thunderbird not responding." This lasts for several seconds.

While looking at the inbox and am clicked on one email and want to scroll down to see other emails, when I try to scroll down the screen gets cloudy and a small rotating circle appears where the pointer is. At the top of the screen a message appears that says "Mozilla Thunderbird not responding". Until the screen clears and the circle disappears, nothing can be done. This gets to be aggravating. What can I do to stop the problem?