Firefox install error

[Sarah J]

Hi

I try to install Firefox with instructions in this page:
mozilla.debian.net

I am running Debian stable (Jessie) and I want to install Firefox
version release

After this row:

sudo apt-get install -t jessie-backports firefox

I get error:

E: The value 'jessie-backports' is invalid for APT::Default-Release as
such a release is not available in the sources

I opened Add / Remove Software app and in Options>Package Sources is

Jessie backports (firefox-release)

I'm newbie so don't know what to try next.

Thanks

-- 
  Sarah J
  sarahjac at fastmail.fm

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