There is a workaround to force older add-on to work with your version of firefox.
1) Type about:config in address bar > Agree the warning > Right-click > New > Boolean > Type extensions.checkCompatibility in boolean value's name and set its value to False.
Basically you are disabling the compatibility check so now you can install any older add-on in any version of firefox. Another way to achieve the same is by editing the extension file itself. But why go the hassle way.