This is a thunderbird addon. If you want to download from Firefox, please right-click the blue button (download now) and download to your Computer. then drag the file into Thunderbird extensions. You can also install it _directly_ from Thunderbird by going to Addons and entering "Quickfolders" in the Search box at the top.
You can install it directly in SeaMonkey. For Postbox, you can also download it from here: https://www.postbox-inc.com/add-ons