Usually the packages available in repositories are older ones. You need some other mechanism to install latest version. Inorder to install latest firefox versions on Ubuntu systems, mozilla has provided a PPA. You could use the PPA to install the latest verssion of Firefox. Also Check For Updates link in Firefox is disabled in Ubuntu. Firefox should only be updated through the package-management system.

 

Step 1:  Visit the PPA's overview page in Launchpad. Look for the heading that reads Adding this PPA to your system and click the Technical details about this PPA link.

Step 2:  Use the Display sources.list entries drop-down box to select the version of Ubuntu you're using.

Step 3: You will get this two lines

deb http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu lucid main
deb-src http://ppa.launchpad.net/ubuntu-mozilla-security/ppa/ubuntu lucid main

Step 4: copy this lines and add to /etc/sources.lst by using the following command in terminal

$ sudo gedit /etc/apt/sources.list

This will open a text editor with sources.lst contents. Add the copied lines to the end portion. Save the file and exit the editor.

Step 5: Back on the PPA's overview page, look for the Signing key heading. You will get 1024R/7EBC211F. To add the key execute the following command in terminal

$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 7EBC211F

Step 6: Now you can install the packages. Run the following commands in terminal

$ sudo apt-get update

Use the below given command to find what all versions are available. And use apt-get install <package_name> to install the required one.

$ sudo apt-cache search firefox
$ sudo apt-get install firefox-3.5

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