I have installed LightSpark Flash Player alternative from what is called the official PPA: https://launchpad.net/~sparkers/+archive/ppa (I mean, I added this repository and then sudo apt-get install lightspark).
But Firefox doesn't feel it. It looks exacly like when I hadn't installed LightSpark. In "Firefox -> Addons -> Plugins" there is no new entry.
What should I do to really use Lightspark after installing it?