From MozillaZine Knowledge Base
When quitting Firefox via “File → Exit”, every open browser window will be closed. This preference determines (along with browser.tabs.warnOnClose) whether or not a warning dialog appears before this happens.
Possible values and their effects
Display a dialog before quitting, warning that all windows will be closed. (Default)
Do not require confirmation from the user when quitting.
First checked in
Has an effect in