Indispensible Rated 5 out of 5 stars
I've been using this extension since it did little more than provide the user with a button to install themes and extensions from one's local directory. It does so much more now. For example, I can make old extensions and themes pretend they are compatible with the current Firefox and with a future Firefox (those developmental builds). I can also disable all my extensions so I can turn them on, one by one, to see which one is causing my Firefox to misbehave.
I make themes. My users write to me often, and I often send them to this extension to repair their Extensions/Themes list. Often this one trick will fix their problems. And then, of course, they can use it to find an extension conflict.
This extension makkes many things easy. I love it.