Some more about this bug:
It seems it's a problem with window focus. If the gnome panel has focus, you can click and manipulate things in the panel, but not anywhere else. If Firefox has focus, you can click and do stuff there, including moving the window, but you can't click on or manipulate or scroll or anything in any other window, *even* if you alt-tab to other windows. Those other windows will highlight and be selected after alt-tabbing, but the *mouse* still can't manipulate them in any way. So it's like the mouse is basically "trapped" and focused on certain windows independently from everything else.
If you alt-F4 to close the focused window, your mouse will be able to operate in the newly-focused window.