How to install previous Firefox version?

I'm using Ubuntu 14.04 and I use the Perapera Chinese add-on for Firefox. It worked up to version 39, but now in version 40 it just stopped working. So I tried to downgrade to version 39 following this link, where there are two options:

1) Choose from the list here, but I have no idea which of the 11 subversions should I use.

2) Download the only version suggested in the link above, i.e. this.

I chose the 2nd option, but after decompressing the file, I have a 'Firefox' folder with lots of files and executables that don't run, just give an error message:

XPCOMGlueLoad error for file (...)/firefox/libxul.so:
libXcomposite.so.1: cannot open shared object file: No such file or directory
Couldn't load XPCOM.

There's a 'run-mozilla.sh', but 'bash run-mozilla.sh' gives:

run-mozilla.sh: Cannot execute .

I REALLY would like to use Perapera add-on to study Chinese. So, is there an easy way to downgrade to Firefox 39? Thanks in advance.

You may have two choice to choose for installing method:

1. Via Mozilla's ftp site (kudos for @Bhikku for sharing the link !). For example we're going to install firefox 50. First, remove any existing version.

   sudo apt-get purge firefox
   

   Then run following command to download firefox 50 source code, which comes as .tar file.

   wget http://ftp.mozilla.org/pub/firefox/releases/50.0/linux-$(uname -m)/en-US/firefox-50.0.tar.bz2
   

   Extract the package.

   tar -xjf firefox-50.0.tar.bz2
   

   Move firefox to /opt directory.

   sudo mv firefox /opt/
   

   Create symlink in order to set the new firefox as default.

   sudo mv /usr/bin/firefox /usr/bin/firefox_old
   sudo ln -s /opt/firefox/firefox /usr/bin/firefox
   

2. Via UbuntuZilla which offer .deb files, to install follow the instruction below.

Remove firefox first :

sudo apt-get purge firefox

Run following command to download firefox :

32 Bit

wget http://downloads.sourceforge.net/project/ubuntuzilla/mozilla/apt/pool/main/f/firefox-mozilla-build/firefox-mozilla-build_39.0.3-0ubuntu1_i386.deb

64 Bit

wget sourceforge.net/projects/ubuntuzilla/files/mozilla/apt/pool/main/f/firefox-mozilla-build/firefox-mozilla-build_39.0.3-0ubuntu1_amd64.deb

Install it :

32 Bit

cd Downloads/ && sudo dpkg -i firefox-mozilla-build_39.0.3-0ubuntu1_i386.deb

64 Bit

cd Downloads/ && sudo dpkg -i firefox-mozilla-build_39.0.3-0ubuntu1_amd64.deb

And firefox should run the 39 version

Quote from ubuntuzilla official wiki page :

This is the home of the Ubuntuzilla project, hosts an APT repository with .deb repacks of the latest official release versions of Mozilla Firefox, Mozilla SeaMonkey, and Mozilla Thunderbird.

In a terminal run:

apt-cache show firefox | grep Version

You will get a list of available versions like in the list below:

Version: 54.0+build3-0ubuntu0.16.04.1
Version: 45.0.2+build1-0ubuntu1

Install the desired version with:

sudo apt-get install firefox=45.0.2+build1-0ubuntu1

Prevent Ubuntu from upgrading to the latest version:

sudo apt-mark hold firefox

While the normal repository often only contains the latest version for older Ubuntu releases, you may be able to find the previous version with the help of Google.

Generalizing from this blog about Firefox 43 on Ubuntu 12.04, you could search for

https://www.google.com/search?q=firefox+$FF_VERSION+$UBUNTU_VERSION+deb

Then

sudo apt-get remove firefox
wget "$URL"
sudo dpkg -i "firefox-....deb"
sudo apt-mark hold firefox

The drawback is that you won't get security upgrades for Firefox, so this is only a temporary solution.