That all "just works" by default.

if you are having problems, make sure they are not self inflicted.

1. Adobe flash is included in the "ubuntu restricted extras" bundle, but
you say that you installed flash "as a firefox plugin" rather than using
the ubuntu provided package. Please describe what actions you took to
install it, and then we can tell you how to reverse the process.

2. Make sure that adobe flash is the only flash player installed, as
installing multiple competing flash players will cause problems
including an inability to view flash content. Remove any competing
players e.g. gnash, swfdec, etc.