It took me a bit to figure out why Firefox was acting so weird.
I tend to have multiple browser windows open, each with multiple tabs (currently 8 windows, probably 70+ tabs overall). If I go to, say, ESPN home’s page, I will rapid-fire right-click all the links that look interesting to me, then read them over time.
In Firefox 3.3.6, if you right-click a link, the first option is “Open link in a new window”, the second is “Open link in a new tab.”
In Firefox 4 Beta: the order is reversed.
I’d ‘internalized’ the order, so all of a sudden, new windows were opening up instead of tabs as I don’t normally pay attention to the options in the right-click menu.