If you've updated to the latest firefox, you might have noticed the "Switch to tab" feature, which checks if you have a website alreay open, and switches to that existing one if you have.
What a stupid thing!
If I open a new tab, its because I want a new tab! It even does it across seperate windows.
To remove this "feature", just install this excellent add-on - Switch to Tab no more.
Phew, back to reasonable web browsing again.