Using the X is closing the interface, but not the process? You can make sure the process is closed by using Exit from the menu, or from the icon on the taskbar. I believe right-clicking on that icon will produce a menu.

GuelphNana said

I close Thunderbird using the X in the upper right hand corner. Later, I wish to open it, but I get a message telling me that TB is still open and that I must restart to get it open again. Can someone help with this issue? Maybe I am closing it incorrectly?

Many thanks, Finitarry. You have solved my problem. Right clicking the icon in the taskbar did not give me the Exit option, but the Menu did!  :-)