Sometimes when I right click on a tab, the menu pops up, but then immediately starts slowly moving in a diagonal towards the lower right. When it hits the right side of the screen, it moves downwards for a while, then pops back up again. This has started for me in the last few days, and still occurs with today's nightly. It doesn't happen all the time, and I'm not quite sure what it takes to reproduce it, but it has happened 5 or so times today already.

The menu sometimes remains interactive, though it is hard to catch it while it is moving!

I had this happen just now when right clicking on a link.