Need to prevent closing while a tab is open

[Steven Schwaber]

I hve used T'bird for years and am very used to closing an open
message by clicking on the x-box at the top right (in fact, it has
become literally a reflex habit). Now with tabs in v.3, I find myself
closing the entire program almost literally each time I finish reading
a message. Firefox has an add on that warns about exiting the program
if there is an open tab. T'bird desperately needs the same function,
but a thorough search does not reveal any such add on or function.

Help, please.

Thanks.

sas