From the apt-get man page,
A specific version of a package can be selected for installation by following the package name with an equals
and the version of the package to select. This will cause that version to be located and selected for install.
Alternatively a specific distribution can be selected by following the package name with a slash and the
version of the distribution or the Archive name (stable, testing, unstable).
sudo apt-get install firefox=5
(note: this fails on my system, probably because I'm using the debian squeeze repos)