You can download video files easily from popular sites like YouTube, Dailymotion, Break.com, etc. using this Firefox add-on - Fast Video Download.
Step 1: Install Fast Video Download on Firefox.
Step 2: Play the YouTube Video that you want to download.
Step 3: Click on the flashing "FVD" icon. (Top right or bottom right of your Firefox browser).
Step 4: Choose the file you want.
Step 5: Save it to your computer.
You are done.
Fast Video Download is also able to download video from a wide range of online video websites, as long you see a flashing "FVD" icon, there is something you can download.