Placing a hold is no guarantee that an update from the normal repository won't overwrite Firefox from Blue Shell. You have to pin the Blue Shell PPA at a higher priority. Here's now:
Code:
sudo add-apt-repository ppa:blue-shell/firefox-kde
echo -e 'Package: *' | sudo tee /etc/apt/preferences.d/blue-shell-firefox > /dev/null
echo -e 'Pin: release o=LP-PPA-blue-shell-firefox-kde' | sudo tee -a /etc/apt/preferences.d/blue-shell-firefox > /dev/null
echo -e 'Pin-Priority: 1000' | sudo tee -a /etc/apt/preferences.d/blue-shell-firefox > /dev/null
sudo apt-get update
sudo apt-get install --no-install-recommends firefox firefox-kde-support