Download Flash and Video is a great download helper tool that lets you download Flash games and Flash videos (YouTube, Facebook, Dailymotion, Google Videos and more) with a single click.
The downloader is very easy to use.
Draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.
FEBE (Firefox Environment Backup Extension) allows you to quickly and easily backup your Firefox extensions. In fact, it goes beyond just backing up -- It will actually rebuild your extensions individually into installable .xpi files.
Reinstates the extensions.checkCompatibility preference without respect to the current application version.
You can capture your screen and record your voice to make screencasts (tutorials, e-lessons, etc.).
HideTab is a simple Firefox extension to hide/unhide individual tabs.