In Firefox, middle mouse button/wheel has been assigned "Open link in new tab" action for a while now, by default.
Since I ran Software Update tonight, this action doesn't work in Firefox anymore. To be specific, middle mouse click doesn't open links anymore. At the same time, it closes tabs just fine (middle-click a tab == 'Close tab').
I don't know the update details, but I remember that Firefox update was there among other things.
I tried running Firefox with a clean profile and middle button did work there, so something must've changed with existing settings.