try right clicking the tar, if your menu has the option to "Expand here" use it for now to get your solution quickly.
As for learning:
open a terminal as ^he said and type "cd /home/<your_user_name_here>/". "cd" is the command to Call Directory, it moves you to whatever directory you want to go to. Once you've found where you downloaded the tar.gz file with the console ("dir" is the command to see a directory btw) type "tar -xzvf <filename>". That will extract the contents into a folder of it's own with a name similar to the name of the tar.gz file. Browse with the console to that directory, hit "dir" to make sure you're in the right directory, you should see a file called "configure" and an "INSTALL" file too("INSTALL file is instructions on how to install). Type the following inside the extracted mozilla tar.gz folder:
a bunch of stuff should go by, it's checking to see if you have the correct programs installed to compile Mozilla.
then when it's done type
this will build the package for your system
this will install the package
if you run into problems just post them, i don't know what you do and don't know lol