Man, it seems to me that the "region collapse" feature of the "Portal" example is quite slow in Firefox (3.5.6), and even creates significant "tearing" of the region content (this example is in Layouts -> Portal). This is worse than even IE, but in the same example FF handles the drag-and-drop of the panels faster than IE. This problem isn't reproduced in the non-GXT portal (or borderLayout) examples.
I spent some time going through the examples, and it seems to me that resizing in general seems unusually slow in FF. However, I recognize that everyone's environment is different, so I thought I would ask if anyone else was experiencing this too, or had some insight on the matter.
By the way, I have Firebug installed but I ran FF in safe mode (i.e. with the "-safe-mode" flag) to prevent any plugins from being loaded. I'm trusting that FF is truly not loading anything.