Sources of the newest firefox (v.42) you can find at following address: https://archive.mozilla.org/pub/firefox/releases/42.0/source/.
After extracting firefox archive go into the directory containing its sources.
Next issue following commands:
cd ..
mkdir ff42
cd ff42
Now you should install following tools and development libraries:
sudo apt-get install libgtk2.0-dev
sudo apt-get install libgconf2-dev
sudo apt-get install libdbus-glib-1-dev
sudo apt-get install yasm
sudo apt-get install libasound2-dev
sudo apt-get install libpulse-dev
sudo apt-get install libgstreamer0.10-dev
sudo apt-get install libgstreamer-plugins-base0.10-dev
sudo apt-get install libxt-dev
Next issue comand that do a configuration tasks and creates makefile files: ../firefox-42.0/configure
Now type in make command and... wait a few hours for compilation of all programmes.
After the compilation all created binaries will be placed in the ff42/dist/bin directory
(exactly symlinks to them). The firefox binary will run without any errors but all other
programmes will not. They generates errors about they couldn't load shared libraries. To solve the problem
create file ff42.conf in directory /etc/ld.so.conf.d/. Into the
ff42.conf file type in the full path to the ff42/dist/bin directory, ex:
/full/path/to/directory/ff42/dist/bin
And at last issue following command:
sudo ldconfig
From now you can cheer up and enjoy with proper working of all compiled programmes.