There exists no official flash package available for the 64bit architecture with Ubuntu and Kubuntu. If you click the add plugin button when visiting a page with flash, you are taken to the official Flash site and will soon notice that there are no available 64bit downloads here either.
Read what to do here
If you are unsure of how to compile a 32bit version of Firefox, you can find out here. We will go over building a 32bit Firefox browser for 64bit systems next time, so if you are daunted by the link above, don't worry, we will tackle this soon. It is pretty straightforward. [Update] This is now covered below. You can skip down to the “32bit Firefox Install” line below.