Firefoxit is an extension to run widgets inside Firefox. It's still very early in development stages and there seem to be almost no widgets but it looks like a promising project.

At the moment you can only show Firefoxit widgets when your inside Firefox. Press F6 to activate the widget screen.

I think the extension needs a bit more on the aesthetics side, getting the widgets to show when not inside the browser, customization and compatibility with other widgets such as Dashboard and Opera.

A new beta is coming soon and it looks like development is continuing so this project could churn out a nice extension to rival Opera's widget support.