If the icon is in Thunderbird is should not be. Anything injected into Thunderbird by an anti virus is in effect malware in it's own right. It compromises the security of the application in infects.

As for add-ons from anti virus companies, what can I say, they have a long history of not working for more than about 6 weeks and rarely in more than one point version of Thunderbird as their are incapable apparently of producing add-ons that do not require binary compatibility. Just remove whatever it is from Thunderbird.

clicking on norton icon has been working ok until last week but now wont open

will not open Norton 360 . it has been working ok until last week