Debian Testing or Debian Sid

You can use /etc/apt/preferences to specify which versions you want on a per-package basis.

To have the latest iceweasel version, use (at your own risk) the following configuration files:

/etc/apt/preferences

Package: iceweasel
Pin: release a=experimental
Pin-Priority: 1000

Package: *
Pin: release a=testing
Pin-Priority: 500

Package: *
Pin: release a=unstable
Pin-Priority: 400

Package: *
Pin: release a=experimental
Pin-Priority: 300

/etc/apt/source.list

deb http://http.debian.net/debian testing         main contrib non-free
deb http://security.debian.org/   testing/updates main
deb http://http.debian.net/debian unstable        main contrib non-free
deb http://http.debian.net/debian experimental    main contrib

But beware the the apt_preferences(5) manpage warns

Packages included in a specific release aren't tested in (and therefore don't always work as expected in) older or newer releases, or together with other packages from different releases. You have been warned.

I don't understand why you are doing here. Why do you have a preferences setting for stable at all if you are running a stable system? As far I know, no preferences setting is necessary for stable in that case.

You don't explicitly say whether you are running a stable system (you really should say so), but if you are not, then I really have no idea what you are doing.

And if the release is on stable, then the usual thing to do for testing and unstable is to set their preferences to less than 100. I usually use 50.

And if you want to downgrade to stable, just do the following (assuming sane settings like the ones above) to downgrade pkgname1 and pkgname2:

apt-get install pkgname1/stable pkgname2/stable

This sets the specified packages to the target release stable.

Incidentally, mixing testing and/or unstable packages with an unstable system is generally a bad idea unless you know what you are doing. Some of the time it is Ok, but most of the time you need to use backports, either from Debian, or self-made.