From MozillaZine Knowledge Base

The current description reads:

When quitting Firefox via “File → Exit”, every open browser window will be closed. This preference determines whether or not a warning dialog appears before this happens.

However, in Firefox 3 (3.0rc2 to be precise) it is used as a switch which describes whether browser.tabs.warnOnClose applies when quitting or not. In other words, this setting is not taken into account if warning when closing multiple tabs is disabled (as described in [comment 6 of bug 428742]).

I don't see this setting under Firefox 2, so the described behaviour must describe how a development build used to work. If so, there should be no trouble in updating the description.