I actually just reinstalled my Mac OS X (feels a lot faster, by the way) but the thing is, I wanted to make sure all my software was up to date and in the process reinstalled divx. Previously I didn't really have a use for divx, I just installed it on a "just in case" basis, and it surely backfired on me this time.
The version that I installed was the very latest "divx 7" and it installed an addon in firefox that COULD NOT BE REMOVED. Just like those annoying java and microsoft addons, this one could not be removed. I was furious, and it took me a while of searching to find out where all the files were (it does not come with an uninstaller, and the company doesn't have one either). So first of all I use pacifist to find out where all the files were, here's a screenshot. The files are quite diverse, and crawl through your whole system.
All of these files were in the macintosh HD directory as well, which was annoying. But, guess what? I uninstalled most of the files, except for the script (I couldn't find it), but the addon was still there! I finally got a tip from a forum, and it turns out that if you go to Macintosh HD/Library/Application Support/Mozilla/Extensions there's a random string of numbers, and when you open it there the divx components are. I deleted it, and am now a happy camper. Good luck!
Edit: For me then I think it was just one folder of random numbers. And I'm pretty sure I have long deleted that file so I am unable to give the exact string. And as long as you delete the files mentioned in the screenshots. So for example go to macintosh HD/library/frameworks and delete the file named "DivX Toolkit.framework," and go through the whole list deleting the files in quicktime, preference panes, etc. Just for the last highlight folder I mentioned, "scripts," you can't delete and you have to go to the extension folder I mentioned.