Please open the Forms gxt 2.2.0 beta demo in IE8 and in Firefox:

Expand the "Company" or the "Time" combo-boxes and then click on the scroll-bar in order to navigate down through the displayed list. It looks like while in Firefox you only need one click, in IE8 you need to do a double-click or something similar?

I don't see this issue in the 2.1.1 based demo.