I have heard alot about firefox Quantum, I have downloaded it and it is running fine. But the problem is I want to update my existing firefox which came installed by default, so as to have all my history, password etc to be on the latest i.e. Quantum one. Currently, it does not have any browsing history or any other data I had on my old-school browser.
EDIT: As far as Firefox Quantum is concerned, it's now available for all currently supported versions of Ubuntu from the repositories. See Ubuntu Package Search Thanks to Videonauth for pointing that out.
If you simply can't wait to try the latest version of Firefox, The Ubuntu Mozilla Security Team has a PPA at https://launchpad.net/~ubuntu-mozilla-security/+archive/ubuntu/ppa
Although it's advisable to wait until they've finished vetting, if you want to do some testing on a non production system of the bleeding edge Firefox browser you can easily do so.
sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa sudo apt-get update sudo apt-get install firefox
Note: Regardless of how you obtain Firefox Quantum, you'll lose your Ubuntu customization extension (and any other incompatible extensions/add-ons you may be using like the SE AutoReview comments add-on, and Flashblock to name 3). So far it's a very responsive browser. For those of you that are interested in the changes to the extensions there's an interesting article here.
Edit: For those of you (like me) who aren't comfortable with losing the SE AutoReview comments add-on and are unwilling to wait and see if a version compatible with Firefox Quantum comes along, a quick fix is the extension for Google Chrome (if you have that or are willing to install it)