Okay, so my friend Orpheum (he's not from Neowin) and I were talking about how much we hate new windows. So we decided to investigate how we can eliminate new windows from being opened by links in Firefox (even if target=_blank on the link). I did a little investigating in about:config, and I found it!
Open up Firefox, open a new tab, go to about:config (just like about:blank but cooler), then scroll down to the 'browser.link.open_newwindow' value, and change the value to '3' (currently set to '2'). Then close that tab!
Now anything that's supposed to open in a new window (it even works in Flash!) will open in a new tab instead! Here's a screenshot for reference.
http://orpheum.ca/imgs/newwindow-to-tab_hack.jpg