This is a known issue with Plugin2, which has become the default applet plug-in
for all browsers on Snow Leopard and Lion.
There is a work-around:
1. Open the Upload New Media window and wait for the applet to launch.
down Cmnd-Shift and move your cursor over the applet window. You will see a
title bar appear with "JumpLoaderApplet" in it.
3. Still holding down
Cmnd-Shift, click the title bar to detach the applet from the browser page.
After you detach the applet it should work as expected. This works for
FireFox and Safari.
Mac does not support this feature for OSX 10.6.X (and letter) and for all modern browsers (Safari, Chrome, Firefox). This problem occurs because of implementation of Java Machine on Macs.