1. Open Firefox, type "about:config" into the location bar (without quotes)

2. Click "I'll be careful I promise" when the confirmation page comes up.

3. Below "preference name" right click, select new -> boolean.

4. Call it "extensions.checkCompatibility" (without quotes). Set the value to false.

What this does is disables Firefox from checking addon compatibility. So (after the tweak) when you install an older addon to a newer build of firefox, firefox won't check to see if it's compatible with it and will allow you to install.