In theory, this is a nice extension. In practice, it periodically changes the focus. For example, if I have one FF3 window open, and I select Tools/Add-ons, after a few seconds, it switches focus back to the main FF window. When I disable the extension, this behavior goes away, and when I re-enable the extension, the focus-switching returns.
I'm guessing this happens when the extension goes to copy the current FF window content, so that it can show the preview in the speed-dial window. I'm also guessing that it doesn't do this for everyone, so it may be that it doesn't play well with one of my other extensions.