I just updated to comebird 4, and open new tab and open new window are the wrong way around 
how can i change it back to the way it was in cometbird 3 like this:
I’m afraid that you probably can’t.
There are many very unwelcome, very unnecessary changes inflicted on FireFox 4.0, including the disappearance of the status bar, that you don’t get a choice about. This may be another one of them.
I’d be delighted to hear that I was wrong, but if you don’t see this as an obvious option in about:config, you probably don’t get a choice.
It seems firefox is following the way “Chrome” does it. I found it annoying that chrome reversed the two options, making the competing browsers work differently, but I am glad that both work the same now. After adjusting to it, it will make a lot of peoples lives easier.
Its now impossible to change the order, but if you prefer the old one.
You may downgrade to 3.6 etc, download links can be found HERE 