Many times, I will go to right click on something in Firefox 4, and the context menu will disappear the second I move my mouse. I can use the arrow keys to select.
This comes and goes. When I was running Maverick, and switched to Minefield, and it began to happen, I wondered if it was something to do with all my customisations and Compiz, but now it is happening in my fairly stock Natty amd64 install.