Maybe you've not got any!
Most Firefox users have things like Adblock Plus (which blocks ads and makes AB's pages load vastly faster). Many people also have extensions (such as Ant Video Downloader) which allow them to save Youtube videos to their hard drive. It's add-ons such as those which make Firefox a really great browser.
Go to Tools > Add-ons, and click on 'Extensions'. If nothing is shown there, or anything there is shown as 'disabled' you can forget about my reference to extensions. (You can also disregard anything related to Java, such as Java Control Panel, which might appear there).
Resetting Firefox takes it back to the way it was when you installed it (while preserving all of your bookmarks and preferences).