It depends on what version of Ubuntu you are using. If you have the new 12.10, look on the launcher and you will see Ubuntu Software Center. Once there search flash and when it comes up click the install button. If you are using a version without the side launcher, you can search flash in Synaptics package manager. I don't use firefox personally, but I believe that would be under preferences in firefox.
Edited by Hadnjury, 10 November 2012 - 09:37 PM.