Try the following: Uninstall Divx plugin from the Tools>addon menu, then Quit Firefox. Open Finder and search for the following folder:

Macintosh HD>Library>Application support>Mozilla>Extensions

There should be at least two folders you can explore (their name is a big bunch of letters and numbers) that contain divx related files, find them and destroy this two folders before restarting Firefox. At least it worked for me!

I tried this and everything that dynamite linked to in another help topic, but it will always keep reinstalling itself. There must be some way of getting rid of this? This is fraudulent by Divx

Interestingly enough, I was able to uninstall the Divx extensions just last week.

I'm not sure what changed, but this did, if memory serves, follow a permission repair and a directory rebuild with Disc Warrior. I had also done a hard shut down and restart. Could be a combination of all of those factors cleaned up something that was wingle wangled (high tech term)

Fri

I was hesitant to delete folders whose purpose I couldn't determine. The Firefox menu "Help/Troubleshooting Information" provided the specific "big bunch of letters and numbers" associated with the DivX plug-in. Sure enough, I found it one level down in the folder that Vegju recommended. I quit Firefox, deleted the folder with the name that matched what I found in the Troubleshooting Information, and when I restarted Firefox the DivX plug-in no longer appeared.