Gtk-Warning **: Cannot open display

[Barnaby Scott]

Oh dear!

I thought I was getting somewhere, but progress is painfully slow until 
I can istall a browser and get it to work, so that I can look up 
solutions to the avalanche of 'challenges' that seem to befalling this 
newbie!

So, I installed Firefox from ports, having made sure everything was bang 
up to date. Evrything seemed to go perfectly well, but lo and behold, 
first attempt to use it and I get this:

(firefox-bin:582): Gtk-Warning **: Cannot open display

What the..?

I have searched for this problem and found plenty of references similar 
error messages, but none of it seems to apply in my case. I am not 
trying to run Firefox as root, I am not doing it from a remote terminal, 
I am not standing in a bucket of water, I have the computer plugged in.

Any clues?