Install firefox using this command:

sudo snap install firefox --edge

... and start using it. Keep an eye on bugs.

Thanks for the reply - yes am very familiar with installing and using Ubunt/Linux etc.. Are there any particular discussion channels recommended for testing issues ?

You can interact with other Firefox Nightly users and give your feedback to Mozilla staff in the Nightly Matrix room on chat.mozilla.org.

Thanks! Exactly what I was looking for.

Thanks for the reply - yes am very familiar with installing and using Ubunt/Linux etc.. Are there any particular discussion channels recommended for testing issues ?

The Nightly test channel of the Firefox Linux tarballs from Mozilla https://www.firefox.com/download/all/desktop-nightly/linux64/ generally gets checkins and updates twice a day in testing. This can sometimes mean issues or unfinished things until fixed, finished or reverted. If you have not used a tarball version before, make sure the Firefox folder has read/write permissions for the user to get internal Firefox updates from Mozilla.

The Nightly builds threads in https://forums.mozillazine.org/viewforum.php?f=23 can give you an idea of what has been fixed or partial fixed in that build.

And thanks James for the further info and link to the forum.