Fortunately Mozilla FireFox 1.5 lets you get back in control.
Select the following options from the menu: Tools > Options > Tabs
Force links that open new windows to open in:
- the same tab/window as the link
- a new tab
I've selected the option the same tab/window as the link, which is what you expect from normal links. You can still open links in a new tab using the third mouse button or clicking with the scroll wheel.