Q:

How do I install the 64bit version of Firefox without losing any of my data?

Firefox 43 comes with a 64bit version. But the one delivered with Ubuntu is 32bit.
Download Firefox link
but in this link there is an option to download 64bit version but its in tar.gz format. Does that mean firefox gets automatically updated and its resources gets added to the software sources or do I have to download again when a new version comes?

A:

The answers provided to you in comments by people such as mchid are dead wrong.
64 bit Firefox for Linux has existed for some time.  What's new is 64 bit support for Windows.
You should use apt to list what packages you have installed, grep for Firefox, and see which architecture you have installed.  If it's x86_64, you're good.  If not, you should uninstall and sudo apt-get install Firefox-x86-64 (or whatever it is called in apt).

