In an overall Windows user experience? Definitely moving windows between screens. Selecting tabs is a pretty niche and uncommon feature across the OS.
It's not about which action is more common, its about the fact that the titlebar drag gesture is universal cross windows 7/8 and is a key element of the user experience.
There are exactly two apps I've used in the entire history of windows 7 that broke this interaction: Chrome and FF. The fault is on those browsers breaking expected behavior, not Opera for doing what windows users expect. The FF UX team admitted that they overlooked this but its understandable that they're keeping it as users have come to expect it.
Multiscreen usage is exceedingly rare, except among hn readers. Among that crowd having lots of tabs open is the norm. So, the subsegment of multiscreen users who don't use a lot of tabs is probably vanishingly small. Myself I'm a windows user a majority of the time and I strongly prefer chrome's behavior.
Another thing lots of applications with custom title bars get wrong: double-clicking the top-left corner of the window (where the icon is, usually) should close the application. (Firefox is an offender here, too, though Chrome is not.)
I had no idea this is a thing, which is.. surprising to say the least. That explains a lot of what I thought were crashes, but wow is that user hostile.
I'm really hesitant to say Firefox gets it "wrong" even if it is inconsistent.
It's not about which action is more common, its about the fact that the titlebar drag gesture is universal cross windows 7/8 and is a key element of the user experience. There are exactly two apps I've used in the entire history of windows 7 that broke this interaction: Chrome and FF. The fault is on those browsers breaking expected behavior, not Opera for doing what windows users expect. The FF UX team admitted that they overlooked this but its understandable that they're keeping it as users have come to expect it.