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I have had a report of an error when trying to install the script from the official Firefox add-on site.

It seems like his installation of Firefox was unable to verify the file's hash string (used to check for data loss during download and other corruption detection).

The solution is to download the add-on at the bottom of this page and install it manually as described below.

Manual Installation (Mac)

According to this page, you can "Just download it to the desktop then drag it to the add-ons box, it will install."

Thanks to "Ski" for reporting this, who said "I just dragged the .xpi file you sent to me onto FF's Add-On window (the frame for Extensions). Your add-on then loaded and a screen popped up telling me to reboot FF to complete the installation."

Manual Installation (PC)

Download and save the xpi file eg to your desktop. Double-click on the downloaded file. When Windows prompts for the application to open it with, select Firefox. FF knows what to do and just installs the add-on. Odd that the FF installation doesn't just associate FF with the .xpi file extension.