Linux – Firefox, two Linux machines, one X-server

I have a Linux machine I regularly log on to via ssh (putty) from Windows. I'm running VcXsrv X-server on my Windows desktop. I mainly use this to run a debugger (ddd) and firefox to access our web-based code review system. It's convenient to use the Linux firefox so I can launch it from a script. I normally launch this with "-new-tab" so as not to keep creating new windows.

Now I've got a new update Linux machine which I'm running alongside the old one for the time being. What I've noticed is some odd behaviour: when I launch a new firefox session if there's one already running on either machine, then it uses that.

What I'd like to be able to do is still launch as a new tab but only under the instance of firefox from the current machine, not an instance from a different machine that happens to be displaying on the same X-server.

I've been playing around with various flags including -no-remote, -new-instance and also defining and using specific profiles. However I've been unable to get the desired result. Either I get the behaviour I described above, or I get an error saying something like "Firefox is already running but not responding, please close it".

EDIT: I've been asked to edit this to provide some examples.

OK. In what follows I'll refer to M_OLD and M_NEW. These are separate machines running different versions of Linux with different versions of Firefox (1.5 and 19). I connect to them both via an ssh client called Putty and am forwarding X to an X server on my Windows desktop. There's only one X server involved.

Example 1:

So from my putty session to M_OLD I run:

firefox www.google.com &

and from my M_NEW putty session I run:

firefox www.imdb.com &

then I get one firefox 1.5 window with two tabs. If I do the same in reverse I get the same result but with firefox 19. In other words, as described in a comment below, the first command launches an instance of firefox the second simply tells the existing instance to open a new tab. Even if the instance is on another machine, so long as it's the same X server.

However I don't want this. I want to have separate instances for separate machines. So:

Example 2:

So from my putty session to M_OLD I run:

firefox -no-remote www.google.com &

and from my M_NEW putty session I run:

firefox -no-remote www.imdb.com &

Now I have two separate instances of firefox. However if I then run on M_NEW:

firefox -no-remote www.google.com &

I'll get

Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.

and even if I try

firefox -no-remote -new-instance www.google.com &

or just

firefox -new-instance www.google.com &

then I'll still get the error.

What I really want – and it may not be possible – is to have a separate instance for each machine but if a new URL is launched from that machine it opens a new tab. It seems as though I can only either have one instance with new tabs for each URL, or one instance per machine, but only one at a time.

Hopefully that's clearer.