To disable the plugin compatibility check for Firefox 3.6 follow these steps:
- Open Firefox
- In the address bar type “about:config”
- On the page click I’ll be careful, I promise!
- Right click and select new –> Boolean
- Enter the preference name: extensions.checkCompatibility.3.6 and press OK
- Select False and press OK
Now you can install plugins that aren’t compatible anymore (but might still work).