Here's an example:
I often find valuable videos on line; either as tutorials or for my own personal enjoyment. But I would like to store them off-line on my computer, so that I can access them with out needing a wireless network connection. Because the Firefox Add-on website categorizes all of their add-ons, I can simply look under the Download Management category and browse for something that I like. I can also search for something specific in the search bar a the top.

I found an add-on called DownloadHelper that easily downloads video from internet sources like YouTube.
There are many more add-ons that become very useful and practical tools for you; organize your tasks with ReminderFox; spot light video to keep students from getting distracted with Turn Off the Lights; add any image online quickly to your Picasa account with AddToPicasa; jot down a few notes with QuickFox Notes; just to name a few. Check out all of the add-ons at Firefox Add-on.