Hi,
I'm trying to run firefox remotely (useful to look at the loopback etc). However,
if I have firefox running on my desktop machine, it just opens a new tab in that
window. This means I'm looking at the wrong loopback.

How do I turn of this bizarre firefox 'feature'.

How are you launching firefox. Are you using ssh?

I'm sshing in, and then starting ssh via the terminal

Did you try
?

There is a setting in firefox to open in tabs instead of a new window, but I don't know if this would effect remote session. Preferences -> Tabs (tab). Just to double check what you meant, a tab opened on the remote computers screen instead of launching firefox on the machine you were at. Is that correct?

I don't get what you mean by checking a loopback.

I have launched firefox in a remote ssh session and haven't noticed the problem, but I can't say whether firefox was already running on the remote machine before logging in.

If I understood well, you have to start firefox locally. Then ssh -X to a machine, start another firefox.
Via X message sorcery, firefox detects that it's already running and will open in the same instance rather than opening a new one.
Seems a bit strange but I've seen other people having the same problem. I can't check myself.

I think the problem the OP is seeing is when firefox is running on on the remote machine, for the same user as the one logging in remotely. It could also be a firefox session problem. I would expect the person to have to create a different profile to be able to run two firefox programs destined for two different displays. Some of the current settings on both may conflict with each other (in ~/.mozilla/firefox/).