[tlug] Firefox dead, can't reinstall

[Brian Chandler <brian@example.com>]

On a neighbouring computer, Firefox has sort of died, so I tried 
reinstalling it. The synaptic package manager thingy said "[something] 
Cannot be authenticated", and when I persisted, just failed anyway. I 
found some help on a forum: tried apt-get update, which listed masses of 
locations, quite a few "can't access", then apt-get install firefox, but 
in the end this all failed. The problem seems to be that it cannot 
access security.ubuntu.com.

On this computer, that domain redirects to somethingelse.ubuntu. And 
this is what I get from apt...

brian@Bach ~ $ sudo apt-get update
[sudo] password for brian:
Ign:1 http://packages.linuxmint.com sarah InRelease
Hit:2 http://archive.canonical.com/ubuntu xenial InRelease
Get:3 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 
kB]
Hit:4 http://archive.ubuntu.com/ubuntu xenial InRelease 

Hit:5 http://packages.linuxmint.com sarah Release 

Get:6 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB] 

Get:8 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [102 
kB]
Fetched 306 kB in 1s (160 kB/s)
Reading package lists... Done

I don't understand what all this means, but it seems to work. How can I 
reset the other computer to a state where apt-get will start working again?

Hope this is a coherent question. TIA.