You have a default jre installed in the browser for 10 years already. The problem is it's not always the right version or flavor - Microsoft did a lot of nasty stuff in this direction. But this was a long time ago, that's why I said these days you probably don't need a separate download.
As an up to date example, I tried a yahoo card game this week-end (http://games.yahoo.com/card-games), and I couldn't launch the game in FF without installing the latest JRE plugin. The game in question if you wanna try is "pyramids".