I have done some more testing and have observed the following:
I think the issue has to do with the mouse pointer position w.r.t. to the context menu position.
1. Right click on the tree item, and while holding the right mouse button pressed, move the cursor over the context menu and then let the mouse button go. Now the context menu will always close.
2. Right click on the tree item, and while holding the right mouse button pressed, move the cursor way from the context menu and then let the mouse button go. Now the context menu won't close. This case is illustrated in the following screen capture: http://www.youtube.com/watch?v=PU9nTUUdmHw
Also, this is not just a FireFox issue. On some browsers/OS the right click aligns the context menu with the mouse pointer and for others it doesn't. So if the user happens to move the mouse while the right button is clicked, and the context menu is not under the mouse pointer when the user releases the right button, then the issue arrises.
I have noticed this also. Has there been any resolution to this issue? This has really thrown a wrench in the progress of the payroll solutions site I am creating. If anyone knows how to fix this problem, please let us know. Thank you very much!