I was trying to use Balsamiq through latest Firefox installed in Ubuntu 9.04, but it didn't work. I was getting the following blank page:
While searching for solution, I found the following commands here, which fixed the problem with flash player and Balsamiq worked successfully then.
From terminal, execute the following commands:
sudo sh ./flash10_en.sh
After running the above commands successfully, I can see Balsamiq mockup pop-up window...