Why isn't DivX working?

[Yam]

Ok, so I've had the problem of going on streaming movie sites and never being able to watch videos that require DivX. I've installed it a few times, and it never works. So I'm trying to watch a movie, I DL it again. I install it, the web player plug in is installed, yet the site still says I do not have the required plug in, and will not allow me to watch this movie. It's driving me crazy, and don't understand if it's the program, or it's my computer.

After doing what the question below said, it still didn't do the trick. Any other possible suggestions?

[theres_blr]

in firefox go to tools -> select add on -> another dialogue box open there select plug in below select update which will upgrade all the need plugin and missing player.