In these days of comfortable living we are likely to forget the discomfort that our ancestors took as a matter of course.
The theater of today, regardless of outside winter temperature, is pleasantly warm, both for audience and performers. Such was not always the case.
Even so recently as a century ago, no matter how warmly dressed, the audience welcomed the intermissions between acts to walk about and get the blood back into circulation; those especially cold retired to the lobby or entrance hall of the theater where a large fire burned upon the hearth. The actors congregated about a similar hearth in the greenroom.
In France, the name for hearth is foyer, and the sense of this word gradually came to include the large hall or room in which the hearth was located.
The need for the hearth and the hearth itself vanished long ago, but the name was borrowed into English usage for both the greenroom and the lobby.