Do it:
$ bzip2 -dc firefox-4.0b1.tar.bz2 | tar -xvf -
You will probably unzip the installer source folder.
If it is an installer, go to run it and install firefox.
If this is the source code, you can probably compile and install it:
$ ./configure $ make $ sudo make install
Otherwise, follow the readme file or instructions that you will find after unpacking the BZIP2 file.
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