[Edit Description]
What it does is when you go to a Firefox add-on page (only on the official website) and it's for older versions, clicking this will force the dialog box to come up. Note you need either Nightly Tester Tools or an about:config tweak to actually let you install it though.
Source (ofmyowncreation.com)
[Edit Javascript]
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javascript%3Ajavascript%3Avar%2520baseURL%2520%3D%27https%3A//addons.mozilla.org%27%3B%2520var%2520str%3Dlocation.href%3B%2520location.href%3DbaseURL+%27/en-US/firefox/downloads/latest/%27+str.substring%2847%2C51%29
force install firefox add-on addon plugin addons older
