Why does Shockwave Flash add-on prevent multiple clicks in a field?

Using Firefox 19.0 and Shockwave Flash 11.6.602.167 (11.6.r602). Mac OSX 10.8.2.

Problem is that I can left-click once in a field and enter data, but if I click a second time, the field will no longer accept input. I have to click outside the field first, and then I can click inside the field to continue entering data.

For example, I click inside a field to enter my user ID. But if I move the mouse back to the first character and then left-click, it will not allow me to enter data. Disabling Shockwave resolves the problem.

This problem occurs on two separate machines - Macbook Pro and Macbook Air.