How to downgrade Firefox from 30 to 28?

Run

apt-cache show firefox | grep Version

to get a list of available Firefox versions. On my 14.04 it results:

Version: 32.0+build1-0ubuntu0.14.04.1
Version: 28.0+build2-0ubuntu2

Next tell APT exactly which version to install:

sudo apt-get install firefox=28.0+build2-0ubuntu2

Next you have to tell APT not to upgrade to the newer version again. According to https://askubuntu.com/a/18656/22865 there are many ways to do it, I chose:

sudo apt-mark hold firefox

Once you want to let go of your Firefox 28 and follow the usual releases again, just do:

sudo apt-mark unhold firefox
sudo apt-get upgrade

Alternatively, you can list the available versions with:

apt-cache policy firefox

PS: Firefox stores all its settings in the ~/.mozilla folder. This folder is not changed by above commands, so all your settings should persist. If you want to be on the safe side, backup you settings before you downgrade:

cp -ra ~/.mozilla ~/.mozilla.backup

Try removing it first by:

sudo apt-get purge firefox

Then you can install using firefox-mozilla-build_28.0-0ubuntu1_i386.deb