Enable Incompatible Add-Ons in Firefox

1. Open Firefox.
2. In the Address field, type about:config and hit Enter.
3. Click on the disclaimer “I’ll be careful, I promise” button.
4. Right-click and from the menu, select New and right after that Boolean.
5. A new, small window will appear on your screen.  Type in extensions.checkCompatibility.5.0 if you have Firefox 5.0. (Replace 5.0 by your version of Firefox).
6. Click OK.
7. When prompted select False and click OK.
8. Restart your browser for the changes to take effect.
9. Run and execute your old, incompatible  addons!