How to install on newer FF versions. Rated 4 out of 5 stars

Step 1. Download the file directly instead of trying to launch the FF installer. "Right click and open in a new tab it will let you download it directly"
Step 2 Open the newly downloaded fire with winrar. "No need to extract anything"
Step 3 Double click the install.rdf to open in notepad++ or whatever you prefer.
Step 4 Change the max version to your current Firefox version. I'll use this install for an example. "Click help then about Firefox.
In my case it is 14.0.1.

Step 5 Save the edited document and when winrar ask to update the archive accept.
Step 6 Open your Add-ons Manager from the tool menu or ctrl+shift+a
Step 7 Click the gear icon on top right next to the search add-ons field and pick the file you just edited.
Step 8 Restart Firefox and enjoy you're newly updated outdated add-on lol.