The DivX Plus Web Player HTML 5 extension can only be disabled through the Firefox Add-ons menu. In order to remove it, you need to uninstall the DivX Plus Software, including the DivX Plus Web Player. However, this extension is completely unrelated to the DivX Browser Bar or the Bing Search override by Conduit.
On a Mac, you can find the "Finder" in your Dock. It's usually on the far left side of the Dock. Once you open Finder, use the left-hand navigation pane to open the Applications folder. In the Applications folder you will see Safari.
If you want to remove extensions from Safari on a Mac, open Safari and then click on the "Safari" menu. Select "Preferences" and then go to "Extensions". You can disable or uninstall extensions from there.
Each browser has their own extensions/add-ons process so extensions need to be disabled or removed separately for each browser. You can find step by step instructions for how to do this here:
For posting here on the DivX Forums, Facebook is one of the many login options. You can use email or other social media logins instead if you don't want to use Facebook. Login is only required for posting and commenting; doing so does not register you for any newsletters or mailings, nor is it shared with any third parties. The only email you would get is a notification that someone has responded to your post here.