Problem solved! And a silly cause, too. I use a second monitor at work, connected through a KVM switch. When connected to the external monitor I also get the other mouse. It was under some books (since I never use it, I'd even forgotten it was there) in my messy office, and the right button...
I'm having the same problem here. It started recently; I'm running a MacBook Pro, but the problem seems unrelated to an external mouse. Occasionally I get an outline as if I'm dragging something in Firefox or Thunderbird, and I can't get rid of this except for killing these applications. This...