Why is my mouse doing this?

[dlf]

In short, some of time when I middle (or right click) the browser thinks I'm left clicking and what ever tab I'm in will open what ever I middle or right clicked -- if i'm playing flash tetris in one tab, middle click to open my email my game of tetris instead loads my email and I have two tabs open to my (g)mail instead of one and a game of tetris in the other.  It only started going this either today, or yesterday.  Using latest cyberfox, and (although I doubt it's needed) Windows 7 Ultimate 64 bit.

[Guest Kloaked]

What mouse is it?

[dlf]

Doubt you can tell exactly it is from the image, really should replace the mouse anyway . . . .