If you really can't wait for the latest version of Firefox to hit the repos - the safest way to install it would probably be to install via snap. Snap is a new packaging format that has been adopted by Ubuntu and can be used alongside .deb.
The snap packages are a complete snapshot of an application, including all of its dependencies.
In order to use snap, you will need to have snapd installed:
Then you can install the latest snapshot of firefox:
Code:
sudo snap install firefox
Then to run firefox:
The "snap" version of firefox will have to be updated via the snap command too.
There are also commands for removing snaps, downgrading to a previously installed version, listing installed snaps, searching for snaps.
Here is a tutorial from a couple of years ago when Ubuntu first introduced the snap package format:
https://itsfoss.com/use-snap-packages-ubuntu-16-04/
The man-pages for snap also contain all the information you'll need to know about using snap.
To view the man-pages for snap use the command: