Hey there!
First off, there is no antivirus that comes with Ubuntu (it's just not needed). Everything however, is locked down and secure by default. This won't interfere with downloads though.
So, what I would do to install flash:
1. Close all open instances of Firefox
2. Open a terminal
3. Type this:
Code:
sudo apt-get -y install ubuntu-restricted-extras
4. Wait for it to download (you might have to type your password)
5. Type exit
6. Start Firefox, enjoy flash!